diff --git a/llvm/test/tools/yaml2obj/macro.yaml b/llvm/test/tools/yaml2obj/macro.yaml
--- a/llvm/test/tools/yaml2obj/macro.yaml
+++ b/llvm/test/tools/yaml2obj/macro.yaml
@@ -58,3 +58,34 @@
   - Name: "[[a]"
   - Name: "[[a[[a]]"
   - Name: "[[a][[a]][[b]]"
+
+## Check that it is possible to set a default value for a macro in the YAML description.
+## Check that we are able to provide an empty default value for a macro.
+# RUN: yaml2obj --docnum=4 %s -o %t4.1
+# RUN: llvm-nm --no-sort %t4.1 | FileCheck %s --check-prefix=DEFAULT
+
+# DEFAULT:      U _foo_
+# DEFAULT-NEXT: U __
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Symbols:
+  - Name: _[[FOO=foo]]_
+  - Name: _[[BAR=]]_
+
+## Check that it is possible to override a default macro value.
+# RUN: yaml2obj --docnum=4 %s -o %t4.2 -DFOO=bar -DBAR=foo
+# RUN: llvm-nm --no-sort %t4.2 | FileCheck %s --check-prefix=OVERRIDE-DEFAULT1
+
+# OVERRIDE-DEFAULT1:      U _bar_
+# OVERRIDE-DEFAULT1-NEXT: U _foo_
+
+# RUN: yaml2obj --docnum=4 %s -o %t4.3 -DFOO=bar=foo -DBAR=foo=bar
+# RUN: llvm-nm --no-sort %t4.3 | FileCheck %s --check-prefix=OVERRIDE-DEFAULT2
+
+# OVERRIDE-DEFAULT2:      U _bar=foo_
+# OVERRIDE-DEFAULT2-NEXT: U _foo=bar_
diff --git a/llvm/tools/yaml2obj/yaml2obj.cpp b/llvm/tools/yaml2obj/yaml2obj.cpp
--- a/llvm/tools/yaml2obj/yaml2obj.cpp
+++ b/llvm/tools/yaml2obj/yaml2obj.cpp
@@ -75,10 +75,22 @@
     if (Buf.startswith("[[")) {
       size_t I = Buf.find_first_of("[]", 2);
       if (Buf.substr(I).startswith("]]")) {
-        StringRef Macro = Buf.substr(2, I - 2);
+        StringRef MacroExpr = Buf.substr(2, I - 2);
+        StringRef Macro;
+        StringRef Default;
+        std::tie(Macro, Default) = MacroExpr.split('=');
+
+        // When the -D option is requested, we use the provided value.
+        // Otherwise we use a default macro value if present.
         auto It = Defines.find(Macro);
-        if (It != Defines.end()) {
-          Preprocessed += It->second;
+        Optional<StringRef> Value;
+        if (It != Defines.end())
+          Value = It->second;
+        else if (!Default.empty() || MacroExpr.endswith("="))
+          Value = Default;
+
+        if (Value) {
+          Preprocessed += *Value;
           Buf = Buf.substr(I + 2);
           continue;
         }