diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index 14410f17ab8a..82f9a021c14e 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -374,6 +374,9 @@ end
   required, with warnings, even if it lacks the BIND(C) attribute.
 * A "mult-operand" in an expression can be preceded by a unary
   `+` or `-` operator.
+* `BIND(C, NAME="...", CDEFINED)` signifies that the storage for an
+  interoperable variable will be allocated outside of Fortran,
+  probably by a C or C++ external definition.
 
 ### Extensions supported when enabled by options
 
diff --git a/flang/include/flang/Parser/parse-tree.h b/flang/include/flang/Parser/parse-tree.h
index 12e35075d2a6..f0b9b682030c 100644
--- a/flang/include/flang/Parser/parse-tree.h
+++ b/flang/include/flang/Parser/parse-tree.h
@@ -1296,10 +1296,13 @@ struct AcImpliedDo {
 };
 
 // R808 language-binding-spec ->
-//        BIND ( C [, NAME = scalar-default-char-constant-expr] )
+//        BIND ( C [, NAME = scalar-default-char-constant-expr ]
+//                 [, CDEFINED ] )
 // R1528 proc-language-binding-spec -> language-binding-spec
-WRAPPER_CLASS(
-    LanguageBindingSpec, std::optional<ScalarDefaultCharConstantExpr>);
+struct LanguageBindingSpec {
+  TUPLE_CLASS_BOILERPLATE(LanguageBindingSpec);
+  std::tuple<std::optional<ScalarDefaultCharConstantExpr>, bool> t;
+};
 
 // R852 named-constant-def -> named-constant = constant-expr
 struct NamedConstantDef {
diff --git a/flang/include/flang/Semantics/symbol.h b/flang/include/flang/Semantics/symbol.h
index 357a4c76d997..cdbe3e39386b 100644
--- a/flang/include/flang/Semantics/symbol.h
+++ b/flang/include/flang/Semantics/symbol.h
@@ -115,10 +115,13 @@ public:
   bool isExplicitBindName() const { return isExplicitBindName_; }
   void set_bindName(std::string &&name) { bindName_ = std::move(name); }
   void set_isExplicitBindName(bool yes) { isExplicitBindName_ = yes; }
+  bool isCDefined() const { return isCDefined_; }
+  void set_isCDefined(bool yes) { isCDefined_ = yes; }
 
 private:
   std::optional<std::string> bindName_;
   bool isExplicitBindName_{false};
+  bool isCDefined_{false};
 };
 
 // Device type specific OpenACC routine information
@@ -814,6 +817,7 @@ public:
   void SetBindName(std::string &&);
   bool GetIsExplicitBindName() const;
   void SetIsExplicitBindName(bool);
+  void SetIsCDefined(bool);
   bool IsFuncResult() const;
   bool IsObjectArray() const;
   const ArraySpec *GetShape() const;
diff --git a/flang/lib/Parser/Fortran-parsers.cpp b/flang/lib/Parser/Fortran-parsers.cpp
index ff01974b549a..13f15c84e579 100644
--- a/flang/lib/Parser/Fortran-parsers.cpp
+++ b/flang/lib/Parser/Fortran-parsers.cpp
@@ -735,7 +735,8 @@ TYPE_PARSER(construct<AccessSpec>("PUBLIC" >> pure(AccessSpec::Kind::Public)) ||
 //        BIND ( C [, NAME = scalar-default-char-constant-expr] )
 // R1528 proc-language-binding-spec -> language-binding-spec
 TYPE_PARSER(construct<LanguageBindingSpec>(
-    "BIND ( C" >> maybe(", NAME =" >> scalarDefaultCharConstantExpr) / ")"))
+    "BIND ( C" >> maybe(", NAME =" >> scalarDefaultCharConstantExpr),
+    (", CDEFINED" >> pure(true) || pure(false)) / ")"))
 
 // R809 coarray-spec -> deferred-coshape-spec-list | explicit-coshape-spec
 // N.B. Bracketed here rather than around references, for consistency with
diff --git a/flang/lib/Parser/unparse.cpp b/flang/lib/Parser/unparse.cpp
index b98aae8e8f7a..13ca2309ad50 100644
--- a/flang/lib/Parser/unparse.cpp
+++ b/flang/lib/Parser/unparse.cpp
@@ -524,7 +524,13 @@ public:
     Word("NULL()");
   }
   void Unparse(const LanguageBindingSpec &x) { // R808 & R1528
-    Word("BIND(C"), Walk(", NAME=", x.v), Put(')');
+    Word("BIND(C");
+    Walk(
+        ", NAME=", std::get<std::optional<ScalarDefaultCharConstantExpr>>(x.t));
+    if (std::get<bool>(x.t)) {
+      Word(", CDEFINED");
+    }
+    Put(')');
   }
   void Unparse(const CoarraySpec &x) { // R809
     common::visit(common::visitors{
diff --git a/flang/lib/Semantics/resolve-names.cpp b/flang/lib/Semantics/resolve-names.cpp
index 7397c3a51b61..17ff12568b06 100644
--- a/flang/lib/Semantics/resolve-names.cpp
+++ b/flang/lib/Semantics/resolve-names.cpp
@@ -315,6 +315,7 @@ private:
   bool IsConflictingAttr(Attr);
 
   MaybeExpr bindName_; // from BIND(C, NAME="...")
+  bool isCDefined_{false}; // BIND(C, NAME="...", CDEFINED) extension
   std::optional<SourceName> passName_; // from PASS(...)
 };
 
@@ -1762,6 +1763,7 @@ Attrs AttrsVisitor::EndAttrs() {
   cudaDataAttr_.reset();
   passName_ = std::nullopt;
   bindName_.reset();
+  isCDefined_ = false;
   return result;
 }
 
@@ -1783,6 +1785,7 @@ void AttrsVisitor::SetBindNameOn(Symbol &symbol) {
       !symbol.attrs().test(Attr::BIND_C)) {
     return;
   }
+  symbol.SetIsCDefined(isCDefined_);
   std::optional<std::string> label{
       evaluate::GetScalarConstantValue<evaluate::Ascii>(bindName_)};
   // 18.9.2(2): discard leading and trailing blanks
@@ -1820,9 +1823,12 @@ void AttrsVisitor::SetBindNameOn(Symbol &symbol) {
 
 void AttrsVisitor::Post(const parser::LanguageBindingSpec &x) {
   if (CheckAndSet(Attr::BIND_C)) {
-    if (x.v) {
-      bindName_ = EvaluateExpr(*x.v);
+    if (const auto &name{
+            std::get<std::optional<parser::ScalarDefaultCharConstantExpr>>(
+                x.t)}) {
+      bindName_ = EvaluateExpr(*name);
     }
+    isCDefined_ = std::get<bool>(x.t);
   }
 }
 bool AttrsVisitor::Pre(const parser::IntentSpec &x) {
@@ -4056,7 +4062,9 @@ void SubprogramVisitor::CreateEntry(
   Attrs attrs;
   const auto &suffix{std::get<std::optional<parser::Suffix>>(stmt.t)};
   bool hasGlobalBindingName{outer.IsGlobal() && suffix && suffix->binding &&
-      suffix->binding->v.has_value()};
+      std::get<std::optional<parser::ScalarDefaultCharConstantExpr>>(
+          suffix->binding->t)
+          .has_value()};
   if (!hasGlobalBindingName) {
     if (Symbol * extant{FindSymbol(outer, entryName)}) {
       if (!HandlePreviousCalls(entryName, *extant, subpFlag)) {
@@ -4440,7 +4448,10 @@ Symbol &SubprogramVisitor::PushSubprogramScope(const parser::Name &name,
     bool hasModulePrefix) {
   Symbol *symbol{GetSpecificFromGeneric(name)};
   if (!symbol) {
-    if (bindingSpec && currScope().IsGlobal() && bindingSpec->v) {
+    if (bindingSpec && currScope().IsGlobal() &&
+        std::get<std::optional<parser::ScalarDefaultCharConstantExpr>>(
+            bindingSpec->t)
+            .has_value()) {
       // Create this new top-level subprogram with a binding label
       // in a new global scope, so that its symbol's name won't clash
       // with another symbol that has a distinct binding label.
@@ -5670,7 +5681,9 @@ bool DeclarationVisitor::Pre(const parser::ProcedureDeclarationStmt &x) {
   const auto &procAttrSpec{std::get<std::list<parser::ProcAttrSpec>>(x.t)};
   for (const parser::ProcAttrSpec &procAttr : procAttrSpec) {
     if (auto *bindC{std::get_if<parser::LanguageBindingSpec>(&procAttr.u)}) {
-      if (bindC->v.has_value()) {
+      if (std::get<std::optional<parser::ScalarDefaultCharConstantExpr>>(
+              bindC->t)
+              .has_value()) {
         if (std::get<std::list<parser::ProcDecl>>(x.t).size() > 1) {
           Say(context().location().value(),
               "A procedure declaration statement with a binding name may not declare multiple procedures"_err_en_US);
diff --git a/flang/lib/Semantics/symbol.cpp b/flang/lib/Semantics/symbol.cpp
index 3eb120fd962f..023ab7b64e4f 100644
--- a/flang/lib/Semantics/symbol.cpp
+++ b/flang/lib/Semantics/symbol.cpp
@@ -375,6 +375,18 @@ void Symbol::SetIsExplicitBindName(bool yes) {
       details_);
 }
 
+void Symbol::SetIsCDefined(bool yes) {
+  common::visit(
+      [&](auto &x) {
+        if constexpr (HasBindName<decltype(&x)>) {
+          x.set_isCDefined(yes);
+        } else {
+          DIE("CDEFINED not allowed on this kind of symbol");
+        }
+      },
+      details_);
+}
+
 bool Symbol::IsFuncResult() const {
   return common::visit(
       common::visitors{[](const EntityDetails &x) { return x.isFuncResult(); },
@@ -422,6 +434,7 @@ llvm::raw_ostream &operator<<(llvm::raw_ostream &os, const EntityDetails &x) {
     os << " type: " << *x.type();
   }
   DumpOptional(os, "bindName", x.bindName());
+  DumpBool(os, "CDEFINED", x.isCDefined());
   return os;
 }
 
diff --git a/flang/test/Semantics/bind-c16.f90 b/flang/test/Semantics/bind-c16.f90
index b9dfb03e35ee..77c1a9160889 100644
--- a/flang/test/Semantics/bind-c16.f90
+++ b/flang/test/Semantics/bind-c16.f90
@@ -84,3 +84,8 @@ module m3
     end
   end interface
 end
+
+!CHECK: cdef01, BIND(C), PUBLIC size=4 offset=0: ObjectEntity type: REAL(4) bindName:cDef01 CDEFINED
+module m4
+  real, bind(c, name='cDef01', cdefined) :: cdef01
+end