Index: COFF/Config.h
===================================================================
--- COFF/Config.h
+++ COFF/Config.h
@@ -122,6 +122,7 @@
   bool DLL = false;
   StringRef Implib;
   std::vector<Export> Exports;
+  std::set<std::string> AllowUnresolved;
   std::set<std::string> DelayLoads;
   std::map<std::string, int> DLLOrder;
   Symbol *DelayLoadHelper = nullptr;
Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -1251,6 +1251,12 @@
   for (auto *Arg : Args.filtered(OPT_incl))
     addUndefined(Arg->getValue());
 
+  // Handle /includeoptional
+  for (auto *Arg : Args.filtered(OPT_incl_optional)) {
+    addUndefined(Arg->getValue());
+    Config->AllowUnresolved.insert(Arg->getValue());
+  }
+
   // Handle /implib
   if (auto *Arg = Args.getLastArg(OPT_implib))
     Config->Implib = Arg->getValue();
Index: COFF/Options.td
===================================================================
--- COFF/Options.td
+++ COFF/Options.td
@@ -167,6 +167,8 @@
 defm demangle : B<"demangle",
     "Demangle symbols in output (default)",
     "Do not demangle symbols in output">;
+def incl_optional : Joined<["/", "-", "/?", "-?"], "includeoptional:">,
+    HelpText<"Add symbol as undefined, but allow it to remain undefined">;
 def kill_at : F<"kill-at">;
 def lldmingw : F<"lldmingw">;
 def output_def : Joined<["/", "-", "/?", "-?"], "output-def:">;
Index: COFF/SymbolTable.cpp
===================================================================
--- COFF/SymbolTable.cpp
+++ COFF/SymbolTable.cpp
@@ -243,6 +243,9 @@
     if (Config->MinGW && handleMinGWAutomaticImport(Sym, Name))
       continue;
 
+    if (Config->AllowUnresolved.count(Name))
+      continue;
+
     // Remaining undefined symbols are not fatal if /force is specified.
     // They are replaced with dummy defined symbols.
     if (Config->ForceUnresolved)
Index: test/COFF/includeoptional.yaml
===================================================================
--- /dev/null
+++ test/COFF/includeoptional.yaml
@@ -0,0 +1,39 @@
+# RUN: yaml2obj < %s > %t.main.obj
+# RUN: yaml2obj < %p/Inputs/include1c.yaml > %t.lib.obj
+# RUN: rm -f %t.lib.lib
+# RUN: llvm-ar cru %t.lib.lib %t.lib.obj
+# RUN: lld-link /out:%t.exe /entry:main /includeoptional:bar /includeoptional:actuallymissing %t.main.obj %t.lib.lib /verbose >& %t.log
+# RUN: FileCheck %s < %t.log
+
+# CHECK: includeoptional.yaml.tmp.main.obj
+# CHECK: includeoptional.yaml.tmp.lib.lib
+# CHECK: includeoptional.yaml.tmp.lib.lib(includeoptional.yaml.tmp.lib.obj) for bar
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     B800000000506800000000680000000050E80000000050E800000000
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          28
+      NumberOfRelocations: 4
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...