Index: llvm/test/tools/llvm-objcopy/COFF/update-section.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-objcopy/COFF/update-section.test
@@ -0,0 +1,31 @@
+# RUN: echo -n AAAB > %t.diff
+# RUN: echo -n AAA > %t.smaller
+# RUN: echo -n AAAAA > %t.larger
+
+# RUN: yaml2obj %s -o %t
+
+# RUN: llvm-objcopy --update-section=.text=%t.diff %t - | obj2yaml | FileCheck %s
+# CHECK: SectionData: '41414142'
+
+# RUN: llvm-objcopy --update-section=.text=%t.smaller %t - | obj2yaml | FileCheck %s --check-prefix=SMALLER
+# SMALLER: SectionData: '41414100'
+
+# RUN: not llvm-objcopy --update-section=.text=%t.larger %t /dev/null 2>&1 | FileCheck %s --check-prefix=TOO-LARGE
+# TOO-LARGE: error: {{.*}}new section cannot be larger than previous section
+
+# RUN: not llvm-objcopy --update-section=.text=%t.noexist %t /dev/null
+
+# RUN: not llvm-objcopy --update-section=.noexist=%t.diff %t /dev/null 2>&1 | FileCheck %s --check-prefix=NO-SECTION
+# NO-SECTION: error: {{.*}}could not find section with name '.noexist'
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .text
+    Characteristics: [  ]
+    Alignment:       4
+    SectionData:     '41414141'
+symbols:
+...
Index: llvm/tools/llvm-objcopy/COFF/COFFObjcopy.cpp
===================================================================
--- llvm/tools/llvm-objcopy/COFF/COFFObjcopy.cpp
+++ llvm/tools/llvm-objcopy/COFF/COFFObjcopy.cpp
@@ -253,6 +253,30 @@
         Characteristics);
   }
 
+  for (StringRef Flag : Config.UpdateSection) {
+    StringRef SecName, FileName;
+    std::tie(SecName, FileName) = Flag.split("=");
+
+    auto BufOrErr = MemoryBuffer::getFile(FileName);
+    if (!BufOrErr)
+      return createFileError(FileName, errorCodeToError(BufOrErr.getError()));
+    auto Buf = std::move(*BufOrErr);
+
+    auto MutableSections = Obj.getMutableSections();
+    auto It = llvm::find_if(MutableSections, [SecName](auto &Sec) {
+      return Sec.Name == SecName;
+    });
+    if (It == MutableSections.end())
+      return createStringError(
+          errc::invalid_argument,
+          "could not find section with name '%s'", SecName.str().c_str());
+    if (It->getContents().size() < Buf->getBufferSize())
+      return createStringError(
+        errc::invalid_argument,
+        "new section cannot be larger than previous section");
+    It->setOwnedContents({Buf->getBufferStart(), Buf->getBufferEnd()});
+  }
+
   if (!Config.AddGnuDebugLink.empty())
     if (Error E = addGnuDebugLink(Obj, Config.AddGnuDebugLink))
       return E;