diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -88,10 +88,12 @@
         common->align = align;
       }
       return s;
-    } else if (!isa<Undefined>(s)) {
-      error("TODO: implement common symbol resolution with other symbol kinds");
-      return s;
     }
+
+    if (isa<Defined>(s))
+      return s;
+    // Common symbols take priority over all non-Defined symbols, so in case of
+    // a name conflict, we fall through to the replaceSymbol() call below.
   }
 
   replaceSymbol<CommonSymbol>(s, name, file, size, align);
diff --git a/lld/test/MachO/common-symbol-resolution.s b/lld/test/MachO/common-symbol-resolution.s
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/common-symbol-resolution.s
@@ -0,0 +1,114 @@
+# REQUIRES: x86
+# RUN: split-file %s %t
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/common.s -o %t/common.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/weak-common.s -o %t/weak-common.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/defined.s -o %t/defined.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/weak-defined.s -o %t/weak-defined.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/libfoo.s -o %t/libfoo.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/calls-foo.s -o %t/calls-foo.o
+
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem -dylib %t/libfoo.o -o %t/libfoo.dylib
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem -dylib %t/libweakfoo.o -o %t/libweakfoo.dylib
+
+# RUN: rm -f %t/defined.a %t/weak-defined-and-common.a
+# RUN: llvm-ar rcs %t/defined.a %t/defined.o
+# RUN: llvm-ar rcs %t/weak-defined-and-common.a %t/weak-defined.o %t/common.o
+
+## The weak attribute appears to have no effect on common symbols. Given two
+## common symbols of the same name, we always pick the one with the larger size,
+## regardless of whether it is weak. Moreover, the resolved symbol in the output
+## file will always be non-weak, even if the winning input symbol definition was
+## weak.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/common.o %t/weak-common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/weak-common.o %t/common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+
+## Defined symbols are the only ones that take precedence over common symbols.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/defined.o %t/common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=DEFINED
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/common.o %t/defined.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=DEFINED
+
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/weak-defined.o %t/common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=WEAK-DEFINED
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/common.o %t/weak-defined.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=WEAK-DEFINED
+
+## Common symbols take precedence over archive symbols.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/defined.a %t/weak-common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/weak-common.o %t/defined.a %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+
+## If an archive has both a common and a defined symbol, the defined one should
+## win.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/weak-defined-and-common.a %t/calls-foo.o -o %t/calls-foo
+# RUN: llvm-objdump --syms %t/calls-foo | FileCheck %s --check-prefix=WEAK-DEFINED
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/calls-foo.o %t/weak-defined-and-common.a -o %t/calls-foo
+# RUN: llvm-objdump --syms %t/calls-foo | FileCheck %s --check-prefix=WEAK-DEFINED
+
+## Common symbols take precedence over dylib symbols.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/libfoo.dylib %t/weak-common.o %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -lSystem %t/weak-common.o %t/libfoo.dylib %t/test.o -o %t/test
+# RUN: llvm-objdump --syms %t/test | FileCheck %s --check-prefix=LARGER-COMMON
+
+# LARGER-COMMON-LABEL: SYMBOL TABLE:
+# LARGER-COMMON-DAG:   [[#%x, FOO_ADDR:]] g     O __DATA,__common _foo
+# LARGER-COMMON-DAG:   [[#FOO_ADDR + 2]]  g     O __DATA,__common _foo_end
+
+# DEFINED-LABEL:       SYMBOL TABLE:
+# DEFINED:             g     F __TEXT,__text _foo
+
+# WEAK-DEFINED-LABEL:  SYMBOL TABLE:
+# WEAK-DEFINED:        w     F __TEXT,__text _foo
+
+#--- order
+## Order is important as we determine the size of a given symbol via the
+## address of the next symbol.
+_foo
+_foo_end
+
+#--- common.s
+.comm _foo, 1
+
+.globl _bar
+_bar:
+
+#--- weak-common.s
+.weak_definition _foo
+.comm _foo, 2
+
+#--- defined.s
+.globl _foo
+_foo:
+  .quad 0x1234
+
+#--- weak-defined.s
+.globl _foo
+.weak_definition _foo
+_foo:
+  .quad 0x1234
+
+#--- libfoo.s
+.globl _foo
+_foo:
+  .quad 0x1234
+
+#--- test.s
+.comm _foo_end, 1
+
+.globl _main
+_main:
+  ret
+
+#--- calls-foo.s
+.comm _foo_end, 1
+
+.globl _main
+_main:
+  callq _foo
+  ret
diff --git a/lld/test/MachO/nonweak-definition-override.s b/lld/test/MachO/nonweak-definition-override.s
--- a/lld/test/MachO/nonweak-definition-override.s
+++ b/lld/test/MachO/nonweak-definition-override.s
@@ -3,6 +3,7 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/libfoo.s -o %t/libfoo.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/nonweakdef.s -o %t/nonweakdef.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/weakdef.s -o %t/weakdef.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/common.s -o %t/common.o
 # RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -dylib %t/libfoo.o -o %t/libfoo.dylib
 
 ## Check that non-weak defined symbols override weak dylib symbols.
@@ -30,6 +31,14 @@
 # NO-WEAK-OVERRIDE-NEXT:  segment section address type addend symbol
 # NO-WEAK-OVERRIDE-EMPTY:
 
+## Check that common symbols take precedence over weak dylib symbols, but do not
+## generate an overridding weak binding.
+# RUN: lld -flavor darwinnew -syslibroot %S/Inputs/MacOSX.sdk -L%t -lfoo %t/common.o -o %t/common -lSystem
+# RUN: llvm-objdump --macho --weak-bind %t/common | FileCheck %s --check-prefix=NO-WEAK-OVERRIDE
+# RUN: llvm-objdump --syms %t/common | FileCheck %s --check-prefix=COMMON
+# COMMON-DAG: g     O __DATA,__common _nonweak_in_dylib
+# COMMON-DAG: g     O __DATA,__common _weak_in_dylib
+
 #--- libfoo.s
 
 .globl _weak_in_dylib, _nonweak_in_dylib
@@ -58,3 +67,12 @@
 
 _main:
   ret
+
+#--- common.s
+
+.globl _main
+.comm _weak_in_dylib, 1
+.comm _nonweak_in_dylib, 1
+
+_main:
+  ret