diff --git a/flang/lib/Evaluate/character.h b/flang/lib/Evaluate/character.h
--- a/flang/lib/Evaluate/character.h
+++ b/flang/lib/Evaluate/character.h
@@ -106,6 +106,7 @@
   static Character REPEAT(const Character &str, ConstantSubscript ncopies) {
     Character result;
     if (!str.empty()) {
+      result.reserve(ncopies * str.size());
       while (ncopies-- > 0) {
         result += str;
       }
diff --git a/flang/lib/Evaluate/fold-character.cpp b/flang/lib/Evaluate/fold-character.cpp
--- a/flang/lib/Evaluate/fold-character.cpp
+++ b/flang/lib/Evaluate/fold-character.cpp
@@ -97,9 +97,16 @@
   } else if (name == "repeat") { // not elemental
     if (auto scalars{GetScalarConstantArguments<T, SubscriptInteger>(
             context, funcRef.arguments())}) {
-      return Expr<T>{Constant<T>{
-          CharacterUtils<KIND>::REPEAT(std::get<Scalar<T>>(*scalars),
-              std::get<Scalar<SubscriptInteger>>(*scalars).ToInt64())}};
+      auto str{std::get<Scalar<T>>(*scalars)};
+      auto n{std::get<Scalar<SubscriptInteger>>(*scalars).ToInt64()};
+      if (static_cast<double>(n) * str.size() >
+          (1 << 20)) { // sanity limit of 1MiB
+        context.messages().Say(
+            "Result of REPEAT() is too large to compute at compilation time (%g characters)"_port_en_US,
+            static_cast<double>(n) * str.size());
+      } else {
+        return Expr<T>{Constant<T>{CharacterUtils<KIND>::REPEAT(str, n)}};
+      }
     }
   } else if (name == "trim") { // not elemental
     if (auto scalar{
diff --git a/flang/test/Evaluate/errors01.f90 b/flang/test/Evaluate/errors01.f90
--- a/flang/test/Evaluate/errors01.f90
+++ b/flang/test/Evaluate/errors01.f90
@@ -137,6 +137,10 @@
     !CHERK: error: Must be a constant value
     integer, parameter :: bad2 = storage_size(y)
   end subroutine
+  subroutine s13
+    !CHECK: portability: Result of REPEAT() is too large to compute at compilation time (1.1259e+15 characters)
+    print *, repeat(repeat(' ', 2**20), 2**30)
+  end subroutine
   subroutine warnings
     real, parameter :: ok1 = scale(0.0, 99999) ! 0.0
     real, parameter :: ok2 = scale(1.0, -99999) ! 0.0