diff --git a/flang-rt/include/flang-rt/runtime/io-error.h b/flang-rt/include/flang-rt/runtime/io-error.h
index 1cef6a208f374..3e8401036f289 100644
--- a/flang-rt/include/flang-rt/runtime/io-error.h
+++ b/flang-rt/include/flang-rt/runtime/io-error.h
@@ -38,6 +38,9 @@ class IoErrorHandler : public Terminator {
   RT_API_ATTRS bool InError() const {
     return ioStat_ != IostatOk || pendingError_ != IostatOk;
   }
+  RT_API_ATTRS bool HasErrorRecovery() const {
+    return (flags_ & (hasIoStat | hasErr)) != 0;
+  }
 
   // For I/O statements that detect fatal errors in their
   // Begin...() API routines before it is known whether they
diff --git a/flang-rt/include/flang-rt/runtime/io-stmt.h b/flang-rt/include/flang-rt/runtime/io-stmt.h
index 9f71d515cb615..7693b60cccfc9 100644
--- a/flang-rt/include/flang-rt/runtime/io-stmt.h
+++ b/flang-rt/include/flang-rt/runtime/io-stmt.h
@@ -729,6 +729,9 @@ class OpenStatementState : public ExternalIoStatementBase {
   RT_API_ATTRS void set_isUnformatted(bool yes = true) {
     isUnformatted_ = yes;
   } // FORM=
+  RT_API_ATTRS void set_mustBeFormatted(bool yes = true) {
+    mustBeFormatted_ = yes;
+  }
 
   RT_API_ATTRS void CompleteOperation();
   RT_API_ATTRS int EndIoStatement();
@@ -743,6 +746,7 @@ class OpenStatementState : public ExternalIoStatementBase {
   OwningPtr<char> path_;
   std::size_t pathLength_{};
   Fortran::common::optional<bool> isUnformatted_;
+  Fortran::common::optional<bool> mustBeFormatted_;
   Fortran::common::optional<Access> access_;
 };
 
diff --git a/flang-rt/lib/runtime/io-api.cpp b/flang-rt/lib/runtime/io-api.cpp
index 6af0121437cd5..c7c15e77c0770 100644
--- a/flang-rt/lib/runtime/io-api.cpp
+++ b/flang-rt/lib/runtime/io-api.cpp
@@ -528,6 +528,9 @@ bool IODEF(SetAdvance)(Cookie cookie, const char *keyword, std::size_t length) {
 
 bool IODEF(SetBlank)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   static const char *keywords[]{"NULL", "ZERO", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
@@ -545,6 +548,9 @@ bool IODEF(SetBlank)(Cookie cookie, const char *keyword, std::size_t length) {
 
 bool IODEF(SetDecimal)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   static const char *keywords[]{"COMMA", "POINT", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
@@ -562,6 +568,9 @@ bool IODEF(SetDecimal)(Cookie cookie, const char *keyword, std::size_t length) {
 
 bool IODEF(SetDelim)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   static const char *keywords[]{"APOSTROPHE", "QUOTE", "NONE", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
@@ -583,6 +592,9 @@ bool IODEF(SetDelim)(Cookie cookie, const char *keyword, std::size_t length) {
 bool IODEF(SetPad)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
   IoErrorHandler &handler{io.GetIoErrorHandler()};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   io.mutableModes().pad = YesOrNo(keyword, length, "PAD", handler);
   return !handler.InError();
 }
@@ -617,6 +629,9 @@ bool IODEF(SetRec)(Cookie cookie, std::int64_t rec) {
 
 bool IODEF(SetRound)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   static const char *keywords[]{"UP", "DOWN", "ZERO", "NEAREST", "COMPATIBLE",
       "PROCESSOR_DEFINED", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
@@ -647,6 +662,9 @@ bool IODEF(SetRound)(Cookie cookie, const char *keyword, std::size_t length) {
 
 bool IODEF(SetSign)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};
+  if (auto *open{io.get_if<OpenStatementState>()}) {
+    open->set_mustBeFormatted();
+  }
   static const char *keywords[]{
       "PLUS", "SUPPRESS", "PROCESSOR_DEFINED", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
@@ -784,6 +802,7 @@ bool IODEF(SetCarriagecontrol)(
     io.GetIoErrorHandler().Crash(
         "SetCarriageControl() called after GetNewUnit() for an OPEN statement");
   }
+  open->set_mustBeFormatted();
   static const char *keywords[]{"LIST", "FORTRAN", "NONE", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
@@ -840,6 +859,7 @@ bool IODEF(SetEncoding)(
     io.GetIoErrorHandler().Crash(
         "SetEncoding() called after GetNewUnit() for an OPEN statement");
   }
+  open->set_mustBeFormatted();
   // Allow the encoding to be changed on an open unit -- it's
   // useful and safe.
   static const char *keywords[]{"UTF-8", "DEFAULT", nullptr};
@@ -872,10 +892,10 @@ bool IODEF(SetForm)(Cookie cookie, const char *keyword, std::size_t length) {
   }
   static const char *keywords[]{"FORMATTED", "UNFORMATTED", "BINARY", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
-  case 0:
+  case 0: // FORM='FORMATTED'
     open->set_isUnformatted(false);
     break;
-  case 1:
+  case 1: // FORM='UNFORMATTED'
     open->set_isUnformatted(true);
     break;
   case 2: // legacy FORM='BINARY' means an unformatted stream
diff --git a/flang-rt/lib/runtime/io-stmt.cpp b/flang-rt/lib/runtime/io-stmt.cpp
index e08088fab4311..c462f60b6b019 100644
--- a/flang-rt/lib/runtime/io-stmt.cpp
+++ b/flang-rt/lib/runtime/io-stmt.cpp
@@ -352,6 +352,17 @@ void OpenStatementState::CompleteOperation() {
     // Set default format (C.7.4 point 2).
     unit().isUnformatted = unit().access != Access::Sequential;
   }
+  if (unit().isUnformatted.value_or(false) && mustBeFormatted_) {
+    // This is an unformatted unit, but the OPEN statement contained at least
+    // one specifier that is not permitted unless the unit is formatted
+    // (e.g., BLANK=).  Programs that want to detect this error (i.e., tests)
+    // should be informed about it, but don't crash the program otherwise
+    // since most other compilers let it slide.
+    if (HasErrorRecovery()) {
+      SignalError("FORM='UNFORMATTED' is not allowed with OPEN specifiers that "
+                  "apply only to formatted units");
+    }
+  }
   if (!wasExtant_ && InError()) {
     // Release the new unit on failure
     set_destroy();