diff --git a/lld/MachO/CMakeLists.txt b/lld/MachO/CMakeLists.txt
--- a/lld/MachO/CMakeLists.txt
+++ b/lld/MachO/CMakeLists.txt
@@ -7,6 +7,7 @@
   Driver.cpp
   InputFiles.cpp
   InputSection.cpp
+  OutputSection.cpp
   OutputSegment.cpp
   SymbolTable.cpp
   Symbols.cpp
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -10,6 +10,7 @@
 #include "Config.h"
 #include "InputFiles.h"
 #include "OutputSegment.h"
+#include "OutputSection.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "Target.h"
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -43,7 +43,7 @@
 
 #include "InputFiles.h"
 #include "InputSection.h"
-#include "OutputSegment.h"
+#include "OutputSection.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "Target.h"
diff --git a/lld/MachO/InputSection.h b/lld/MachO/InputSection.h
--- a/lld/MachO/InputSection.h
+++ b/lld/MachO/InputSection.h
@@ -19,7 +19,7 @@
 
 class InputFile;
 class InputSection;
-class OutputSegment;
+class OutputSection;
 class Symbol;
 
 struct Reloc {
@@ -35,6 +35,8 @@
   virtual size_t getSize() const { return data.size(); }
   virtual uint64_t getFileSize() const { return getSize(); }
   uint64_t getFileOffset() const;
+  uint64_t getVA() const;
+
   // Don't emit section_64 headers for hidden sections.
   virtual bool isHidden() const { return false; }
   // Unneeded sections are omitted entirely (header and body).
@@ -42,19 +44,16 @@
   virtual void writeTo(uint8_t *buf);
 
   InputFile *file = nullptr;
-  OutputSegment *parent = nullptr;
   StringRef name;
   StringRef segname;
 
-  ArrayRef<uint8_t> data;
+  OutputSection *parent = nullptr;
+  uint64_t outSecOff = 0;
 
-  // TODO these properties ought to live in an OutputSection class.
-  // Move them once available.
-  uint64_t addr = 0;
   uint32_t align = 1;
-  uint32_t sectionIndex = 0;
   uint32_t flags = 0;
 
+  ArrayRef<uint8_t> data;
   std::vector<Reloc> relocs;
 };
 
diff --git a/lld/MachO/InputSection.cpp b/lld/MachO/InputSection.cpp
--- a/lld/MachO/InputSection.cpp
+++ b/lld/MachO/InputSection.cpp
@@ -22,9 +22,11 @@
 std::vector<InputSection *> macho::inputSections;
 
 uint64_t InputSection::getFileOffset() const {
-  return parent->fileOff + addr - parent->firstSection()->addr;
+  return parent->getFileOffset() + outSecOff;
 }
 
+uint64_t InputSection::getVA() const { return parent->addr + outSecOff; }
+
 void InputSection::writeTo(uint8_t *buf) {
   memcpy(buf, data.data(), data.size());
 
@@ -32,18 +34,18 @@
     uint64_t va = 0;
     if (auto *s = r.target.dyn_cast<Symbol *>()) {
       if (auto *dylibSymbol = dyn_cast<DylibSymbol>(s)) {
-        va = in.got->addr - ImageBase + dylibSymbol->gotIndex * WordSize;
+        va = in.got->getVA() - ImageBase + dylibSymbol->gotIndex * WordSize;
       } else {
         va = s->getVA();
       }
     } else if (auto *isec = r.target.dyn_cast<InputSection *>())
-      va = isec->addr;
+      va = isec->getVA();
     else
       llvm_unreachable("Unknown relocation target");
 
     uint64_t val = va + r.addend;
     if (1) // TODO: handle non-pcrel relocations
-      val -= addr - ImageBase + r.offset;
+      val -= getVA() - ImageBase + r.offset;
     target->relocateOne(buf + r.offset, r.type, val);
   }
 }
diff --git a/lld/MachO/OutputSection.h b/lld/MachO/OutputSection.h
new file mode 100644
--- /dev/null
+++ b/lld/MachO/OutputSection.h
@@ -0,0 +1,59 @@
+//===- OutputSection.h ------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_MACHO_OUTPUT_SECTION_H
+#define LLD_MACHO_OUTPUT_SECTION_H
+
+#include "InputSection.h"
+#include "lld/Common/LLVM.h"
+#include "llvm/ADT/MapVector.h"
+
+namespace lld {
+namespace macho {
+
+class InputSection;
+class OutputSegment;
+
+class OutputSection {
+public:
+  const InputSection *firstSection() const { return inputs.front(); }
+  const InputSection *lastSection() const { return inputs.back(); }
+
+  // These accessors will only be valid after finalizing the section
+  uint64_t getFileOffset() const;
+  size_t getSize() const { return size; }
+  size_t getFileSize() const { return fileSize; }
+  bool isHidden() const { return hidden; }
+
+  void addInput(InputSection *input);
+  void finalize();
+
+  void writeTo(uint8_t *buf) const;
+
+  StringRef name;
+  OutputSegment *parent = nullptr;
+  std::vector<InputSection *> inputs;
+
+  uint32_t index = 0;
+
+  uint64_t addr = 0;
+  uint32_t align = 1;
+  uint32_t flags = 0;
+
+private:
+  void mergeFlags(uint32_t inputFlags);
+
+  bool hidden = false;
+  size_t size = 0;
+  size_t fileSize = 0;
+};
+
+} // namespace macho
+} // namespace lld
+
+#endif
diff --git a/lld/MachO/OutputSection.cpp b/lld/MachO/OutputSection.cpp
new file mode 100644
--- /dev/null
+++ b/lld/MachO/OutputSection.cpp
@@ -0,0 +1,72 @@
+//===- OutputSection.cpp --------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "OutputSection.h"
+#include "OutputSegment.h"
+#include "lld/Common/ErrorHandler.h"
+#include "lld/Common/Memory.h"
+#include "llvm/BinaryFormat/MachO.h"
+
+using namespace llvm;
+using namespace llvm::MachO;
+using namespace lld;
+using namespace lld::macho;
+
+uint64_t OutputSection::getFileOffset() const {
+  return parent->fileOff + addr - parent->firstSection()->addr;
+}
+
+void OutputSection::addInput(InputSection *input) {
+  if (this->inputs.empty()) {
+    this->align = input->align;
+    this->flags = input->flags;
+  } else {
+    this->mergeFlags(input->flags);
+    this->align = std::max(this->align, input->align);
+  }
+
+  this->hidden |= input->isHidden();
+  this->inputs.push_back(input);
+  input->parent = this;
+}
+
+void OutputSection::finalize() {
+  uint64_t addr = this->addr;
+  this->fileSize = 0;
+
+  for (InputSection *i : inputs) {
+    i->outSecOff = alignTo(addr, i->align) - this->addr;
+    addr += i->getSize();
+    this->fileSize += i->getFileSize();
+  }
+  this->size = addr - this->addr;
+}
+
+void OutputSection::writeTo(uint8_t *buf) const {
+  for (InputSection *i : inputs) {
+    i->writeTo(buf);
+    buf += i->getFileSize();
+  }
+}
+
+void OutputSection::mergeFlags(uint32_t inputFlags) {
+  uint8_t sectionFlag = MachO::SECTION_TYPE & inputFlags;
+  if (sectionFlag != (MachO::SECTION_TYPE & this->flags))
+    error("Cannot add merge section; inconsistent type flags " + Twine(sectionFlag));
+
+  uint32_t inconsistentFlags = MachO::S_ATTR_DEBUG | MachO::S_ATTR_STRIP_STATIC_SYMS | MachO::S_ATTR_NO_DEAD_STRIP | MachO::S_ATTR_LIVE_SUPPORT;
+  if ((inputFlags ^ this->flags) & inconsistentFlags)
+    error("Cannot add merge section; cannot merge inconsistent flags");
+
+  // Negate pure instruction presence if any segment isn't pure.
+  uint32_t pureMask = ~(MachO::S_ATTR_PURE_INSTRUCTIONS & inputFlags &this->flags);
+
+  // Merge the rest
+  this->flags |= inputFlags;
+  this->flags &= pureMask;
+}
diff --git a/lld/MachO/OutputSegment.h b/lld/MachO/OutputSegment.h
--- a/lld/MachO/OutputSegment.h
+++ b/lld/MachO/OutputSegment.h
@@ -9,6 +9,7 @@
 #ifndef LLD_MACHO_OUTPUT_SEGMENT_H
 #define LLD_MACHO_OUTPUT_SEGMENT_H
 
+#include "OutputSection.h"
 #include "lld/Common/LLVM.h"
 #include "llvm/ADT/MapVector.h"
 
@@ -23,34 +24,34 @@
 
 } // namespace segment_names
 
+class OutputSection;
 class InputSection;
 
 class OutputSegment {
 public:
-  InputSection *firstSection() const { return sections.front().second.at(0); }
+  typedef llvm::MapVector<StringRef, OutputSection *> SectionMap;
 
-  InputSection *lastSection() const { return sections.back().second.back(); }
+  const OutputSection *firstSection() const { return sections.front().second; }
+
+  const OutputSection *lastSection() const { return sections.back().second; }
 
   bool isNeeded() const {
     return !sections.empty() || name == segment_names::linkEdit;
   }
 
-  void addSection(InputSection *);
+  OutputSection *addSection(InputSection *);
 
-  const llvm::MapVector<StringRef, std::vector<InputSection *>> &
-  getSections() const {
+  const SectionMap &getSections() const {
     return sections;
   }
 
+  uint32_t numNonHiddenSections = 0;
   uint64_t fileOff = 0;
   StringRef name;
-  uint32_t numNonHiddenSections = 0;
   uint32_t maxProt = 0;
   uint32_t initProt = 0;
   uint8_t index;
-
-private:
-  llvm::MapVector<StringRef, std::vector<InputSection *>> sections;
+  SectionMap sections;
 };
 
 extern std::vector<OutputSegment *> outputSegments;
diff --git a/lld/MachO/OutputSegment.cpp b/lld/MachO/OutputSegment.cpp
--- a/lld/MachO/OutputSegment.cpp
+++ b/lld/MachO/OutputSegment.cpp
@@ -33,13 +33,25 @@
   return VM_PROT_READ | VM_PROT_WRITE | VM_PROT_EXECUTE;
 }
 
-void OutputSegment::addSection(InputSection *isec) {
-  isec->parent = this;
-  std::vector<InputSection *> &vec = sections[isec->name];
-  if (vec.empty() && !isec->isHidden()) {
-    ++numNonHiddenSections;
+OutputSection *OutputSegment::addSection(InputSection *input) {
+  OutputSegment::SectionMap::iterator i = this->sections.find(input->name);
+  if (i != this->sections.end()) {
+    auto os = i->second;
+    os->addInput(input);
+    return os;
   }
-  vec.push_back(isec);
+
+  auto *os = make<OutputSection>();
+  os->name = input->name;
+  os->parent = this;
+  os->addInput(input);
+  this->sections[os->name] = os;
+
+  if (!os->isHidden()) {
+    this->numNonHiddenSections++;
+  }
+
+  return os;
 }
 
 static llvm::DenseMap<StringRef, OutputSegment *> nameToOutputSegment;
diff --git a/lld/MachO/Symbols.h b/lld/MachO/Symbols.h
--- a/lld/MachO/Symbols.h
+++ b/lld/MachO/Symbols.h
@@ -81,7 +81,7 @@
 
 inline uint64_t Symbol::getVA() const {
   if (auto *d = dyn_cast<Defined>(this))
-    return d->isec->addr + d->value - ImageBase;
+    return d->isec->getVA() + d->value - ImageBase;
   return 0;
 }
 
diff --git a/lld/MachO/SyntheticSections.cpp b/lld/MachO/SyntheticSections.cpp
--- a/lld/MachO/SyntheticSections.cpp
+++ b/lld/MachO/SyntheticSections.cpp
@@ -106,7 +106,7 @@
   raw_svector_ostream os{contents};
   os << static_cast<uint8_t>(BIND_OPCODE_SET_SEGMENT_AND_OFFSET_ULEB |
                              in.got->parent->index);
-  encodeULEB128(in.got->addr - in.got->parent->firstSection()->addr, os);
+  encodeULEB128(in.got->outSecOff, os);
   for (const DylibSymbol *sym : in.got->getEntries()) {
     // TODO: Implement compact encoding -- we only need to encode the
     // differences between consecutive symbol entries.
@@ -191,11 +191,12 @@
     // TODO support other symbol types
     // TODO populate n_desc
     if (auto defined = dyn_cast<Defined>(entry.sym)) {
+      auto *parent = defined->isec->parent;
       nList->n_strx = entry.strx;
       nList->n_type = N_EXT | N_SECT;
-      nList->n_sect = defined->isec->sectionIndex;
+      nList->n_sect = parent->index;
       // For the N_SECT symbol type, n_value is the address of the symbol
-      nList->n_value = defined->value + defined->isec->addr;
+      nList->n_value = defined->value + defined->isec->outSecOff + parent->addr;
     }
 
     ++nList;
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -38,7 +38,7 @@
 
   void scanRelocations();
   void createHiddenSections();
-  void sortSections();
+  void createOutputSections();
   void createLoadCommands();
   void assignAddresses(OutputSegment *);
   void createSymtabContents();
@@ -125,10 +125,9 @@
 
     for (auto &p : seg->getSections()) {
       StringRef s = p.first;
-      ArrayRef<InputSection *> sections = p.second;
-      for (InputSection *isec : sections)
-        c->filesize += isec->getFileSize();
-      if (sections[0]->isHidden())
+      OutputSection *section = p.second;
+      c->filesize += section->getFileSize();
+      if (section->isHidden())
         continue;
 
       auto *sectHdr = reinterpret_cast<section_64 *>(buf);
@@ -137,16 +136,11 @@
       memcpy(sectHdr->sectname, s.data(), s.size());
       memcpy(sectHdr->segname, name.data(), name.size());
 
-      sectHdr->addr = sections[0]->addr;
-      sectHdr->offset = sections[0]->getFileOffset();
-      sectHdr->align = sections[0]->align;
-      uint32_t maxAlign = 0;
-      for (const InputSection *section : sections)
-        maxAlign = std::max(maxAlign, section->align);
-      sectHdr->align = Log2_32(maxAlign);
-      sectHdr->flags = sections[0]->flags;
-      sectHdr->size = sections.back()->addr + sections.back()->getSize() -
-                      sections[0]->addr;
+      sectHdr->addr = section->addr;
+      sectHdr->offset = section->getFileOffset();
+      sectHdr->align = Log2_32(section->align);
+      sectHdr->flags = section->flags;
+      sectHdr->size = section->getSize();
     }
   }
 
@@ -376,20 +370,27 @@
   }
 }
 
-void Writer::sortSections() {
+void Writer::createOutputSections() {
   llvm::stable_sort(inputSections, SectionComparator());
 
-  // TODO This is wrong; input sections ought to be grouped into
-  // output sections, which are then organized like this.
-  uint32_t sectionIndex = 0;
-  // Add input sections to output segments.
+  // Add input sections to output sections/segments.
   for (InputSection *isec : inputSections) {
     if (isec->isNeeded()) {
-      if (!isec->isHidden())
-        isec->sectionIndex = ++sectionIndex;
       getOrCreateOutputSegment(isec->segname)->addSection(isec);
     }
   }
+
+  // Now that the input sections are sorted, assign the final
+  // output section indices.
+  uint32_t sectionIndex = 0;
+  for (OutputSegment *seg : outputSegments) {
+    for (auto &p : seg->getSections()) {
+      OutputSection *section = p.second;
+      if (!section->isHidden()) {
+        section->index = ++sectionIndex;
+      }
+    }
+  }
 }
 
 void Writer::assignAddresses(OutputSegment *seg) {
@@ -398,13 +399,13 @@
   seg->fileOff = fileOff;
 
   for (auto &p : seg->getSections()) {
-    ArrayRef<InputSection *> sections = p.second;
-    for (InputSection *isec : sections) {
-      addr = alignTo(addr, isec->align);
-      isec->addr = addr;
-      addr += isec->getSize();
-      fileOff += isec->getFileSize();
-    }
+    OutputSection *section = p.second;
+    addr = alignTo(addr, section->align);
+    section->addr = addr;
+    section->finalize();
+
+    addr += section->getSize();
+    fileOff += section->getFileSize();
   }
 }
 
@@ -424,11 +425,10 @@
   uint8_t *buf = buffer->getBufferStart();
   for (OutputSegment *seg : outputSegments) {
     uint64_t fileOff = seg->fileOff;
-    for (auto &sect : seg->getSections()) {
-      for (InputSection *isec : sect.second) {
-        isec->writeTo(buf + fileOff);
-        fileOff += isec->getFileSize();
-      }
+    for (auto &p : seg->getSections()) {
+      OutputSection *section = p.second;
+      section->writeTo(buf + fileOff);
+      fileOff += section->getFileSize();
     }
   }
 }
@@ -438,9 +438,9 @@
   createHiddenSections();
   // Sort and assign sections to their respective segments. No more sections can
   // be created after this method runs.
-  sortSections();
+  createOutputSections();
   // dyld requires __LINKEDIT segment to always exist (even if empty).
-  getOrCreateOutputSegment(segment_names::linkEdit);
+  auto *linkEditSegment = getOrCreateOutputSegment(segment_names::linkEdit);
   // No more segments can be created after this method runs.
   createLoadCommands();
 
@@ -460,8 +460,8 @@
 
   // Now that __LINKEDIT is filled out, do a proper calculation of its
   // addresses and offsets. We don't have to recalculate the other segments
-  // since sortSections() ensures that __LINKEDIT is the last segment.
-  assignAddresses(getOutputSegment(segment_names::linkEdit));
+  // since createOutputSections() ensures that __LINKEDIT is the last segment.
+  assignAddresses(linkEditSegment);
 
   openFile();
   if (errorCount())
diff --git a/lld/test/MachO/section-merge.s b/lld/test/MachO/section-merge.s
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/section-merge.s
@@ -0,0 +1,46 @@
+# REQUIRES: x86
+# RUN: mkdir -p %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %p/Inputs/libhello.s \
+# RUN:   -o %t/libhello.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %p/Inputs/libgoodbye.s \
+# RUN:   -o %t/libgoodbye.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s \
+# RUN:   -o %t/main.o
+# RUN: lld -flavor darwinnew -o %t/output %t/libgoodbye.o %t/libhello.o %t/main.o
+# RUN: llvm-readobj -symbols %t/output | FileCheck %s
+
+# CHECK:          Name: _goodbye_world
+# CHECK-NEXT:     Extern
+# CHECK-NEXT:     Type: Section (0xE)
+# CHECK-NEXT:     Section: __cstring (0x2)
+# CHECK-NEXT:     RefType:
+# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Value: 0x[[#%X,BASE:]]
+
+# CHECK:          Name: _hello_world
+# CHECK-NEXT:     Extern
+# CHECK-NEXT:     Type: Section (0xE)
+# CHECK-NEXT:     Section: __cstring (0x2)
+# CHECK-NEXT:     RefType:
+# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Value: 0x[[#BASE + 0x10]]
+
+# CHECK:          Name: _main
+# CHECK-NEXT:     Extern
+# CHECK-NEXT:     Type: Section (0xE)
+# CHECK-NEXT:     Section: __text (0x1)
+# CHECK-NEXT:     RefType:
+# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     ]
+# CHECK-NEXT:     Value: 0x[[#BASE - 0x8]]
+
+.section __TEXT,__text
+.global _goodbye_world
+.global _hello_world
+.global _main
+
+_main:
+  mov $0, %rax
+  ret