diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index f25f0d1e0ca39..e84bceee5ca7c 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -411,6 +411,8 @@ end
   is accepted before an array specification (`ch*3(2)`) as well
   as afterwards.
 * A zero field width is allowed for logical formatted output (`L0`).
+* `OPEN(..., FORM='BINARY')` is accepted as a legacy synonym for
+  the standard `OPEN(..., FORM='UNFORMATTED', ACCESS='STREAM')`.
 
 ### Extensions supported when enabled by options
 
diff --git a/flang/lib/Semantics/check-io.cpp b/flang/lib/Semantics/check-io.cpp
index 3c99163c1f134..42c3b9e11efc1 100644
--- a/flang/lib/Semantics/check-io.cpp
+++ b/flang/lib/Semantics/check-io.cpp
@@ -920,7 +920,7 @@ void IoChecker::CheckStringValue(IoSpecKind specKind, const std::string &value,
       {IoSpecKind::Decimal, {"COMMA", "POINT"}},
       {IoSpecKind::Delim, {"APOSTROPHE", "NONE", "QUOTE"}},
       {IoSpecKind::Encoding, {"DEFAULT", "UTF-8"}},
-      {IoSpecKind::Form, {"FORMATTED", "UNFORMATTED"}},
+      {IoSpecKind::Form, {"FORMATTED", "UNFORMATTED", "BINARY"}},
       {IoSpecKind::Pad, {"NO", "YES"}},
       {IoSpecKind::Position, {"APPEND", "ASIS", "REWIND"}},
       {IoSpecKind::Round,
diff --git a/flang/runtime/io-api.cpp b/flang/runtime/io-api.cpp
index 9dfa09ab332c2..dc3f6f87fc21b 100644
--- a/flang/runtime/io-api.cpp
+++ b/flang/runtime/io-api.cpp
@@ -887,7 +887,7 @@ bool IODEF(SetForm)(Cookie cookie, const char *keyword, std::size_t length) {
     io.GetIoErrorHandler().Crash(
         "SetForm() called after GetNewUnit() for an OPEN statement");
   }
-  static const char *keywords[]{"FORMATTED", "UNFORMATTED", nullptr};
+  static const char *keywords[]{"FORMATTED", "UNFORMATTED", "BINARY", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
     open->set_isUnformatted(false);
@@ -895,6 +895,10 @@ bool IODEF(SetForm)(Cookie cookie, const char *keyword, std::size_t length) {
   case 1:
     open->set_isUnformatted(true);
     break;
+  case 2: // legacy FORM='BINARY' means an unformatted stream
+    open->set_isUnformatted(true);
+    open->set_access(Access::Stream);
+    break;
   default:
     open->SignalError(IostatErrorInKeyword, "Invalid FORM='%.*s'",
         static_cast<int>(length), keyword);