diff --git a/flang/lib/Frontend/FrontendAction.cpp b/flang/lib/Frontend/FrontendAction.cpp
--- a/flang/lib/Frontend/FrontendAction.cpp
+++ b/flang/lib/Frontend/FrontendAction.cpp
@@ -51,6 +51,13 @@
 
   Fortran::parser::Options parserOptions =
       this->instance().invocation().fortranOpts();
+  // Set the fixed form flag based on the file extension
+  auto pathDotIndex{currentInputPath.rfind(".")};
+  if (pathDotIndex != std::string::npos) {
+    std::string pathSuffix{currentInputPath.substr(pathDotIndex + 1)};
+    parserOptions.isFixedForm =
+        pathSuffix == "f" || pathSuffix == "F" || pathSuffix == "ff";
+  }
 
   // Prescan. In case of failure, report and return.
   ci.parsing().Prescan(currentInputPath, parserOptions);
diff --git a/flang/lib/Frontend/FrontendOptions.cpp b/flang/lib/Frontend/FrontendOptions.cpp
--- a/flang/lib/Frontend/FrontendOptions.cpp
+++ b/flang/lib/Frontend/FrontendOptions.cpp
@@ -13,9 +13,11 @@
 
 InputKind FrontendOptions::GetInputKindForExtension(llvm::StringRef extension) {
   return llvm::StringSwitch<InputKind>(extension)
-      // TODO: Should match the list in flang/test/lit.cfg.py
+      // Note: keep this list in-sync with flang/test/lit.cfg.py
+      // TODO: Add Cuda Fortan files (i.e. `*.cuf` and `*.CUF`).
       // FIXME: Currently this API allows at most 9 items per case.
       .Cases("f", "F", "f77", "f90", "F90", "f95", "F95", "ff95", "f18", "F18",
           Language::Fortran)
+      .Cases("ff", "FOR", "for", "f77", "ff90", "fpp", "FPP", Language::Fortran)
       .Default(Language::Unknown);
 }
diff --git a/flang/test/Flang-Driver/Inputs/fixed-form-test.f b/flang/test/Flang-Driver/Inputs/fixed-form-test.f
new file mode 100644
--- /dev/null
+++ b/flang/test/Flang-Driver/Inputs/fixed-form-test.f
@@ -0,0 +1,2 @@
+      program A
+      end
\ No newline at end of file
diff --git a/flang/test/Flang-Driver/Inputs/free-form-test.f90 b/flang/test/Flang-Driver/Inputs/free-form-test.f90
new file mode 100644
--- /dev/null
+++ b/flang/test/Flang-Driver/Inputs/free-form-test.f90
@@ -0,0 +1,2 @@
+program A
+end
\ No newline at end of file
diff --git a/flang/test/Flang-Driver/fixed-free-detection.f90 b/flang/test/Flang-Driver/fixed-free-detection.f90
new file mode 100644
--- /dev/null
+++ b/flang/test/Flang-Driver/fixed-free-detection.f90
@@ -0,0 +1,38 @@
+! Ensure the driver behaves differently on a fixed form file, to a free form file, based on the file extension.
+
+! REQUIRES: new-flang-driver
+
+!--------------------------
+! FLANG DRIVER (flang-new)
+!--------------------------
+! RUN: %flang-new -E %S/Inputs/free-form-test.f90  2>&1 | FileCheck %s --check-prefix=FREEFORM
+! RUN: %flang-new -E %S/Inputs/fixed-form-test.f  2>&1 | FileCheck %s --check-prefix=FIXEDFORM
+! RUN: %flang-new -E %S/Inputs/free-form-test.f90 %S/Inputs/fixed-form-test.f  2>&1 | FileCheck %s --check-prefix=MULTIPLEFORMS
+
+!-----------------------------------------
+! FRONTEND FLANG DRIVER (flang-new -fc1)
+!-----------------------------------------
+! RUN: %flang-new -fc1 -E %S/Inputs/free-form-test.f90  2>&1 | FileCheck %s --check-prefix=FREEFORM
+! RUN: %flang-new -fc1 -E %S/Inputs/fixed-form-test.f  2>&1 | FileCheck %s --check-prefix=FIXEDFORM
+! RUN: %flang-new -fc1 -E %S/Inputs/free-form-test.f90 %S/Inputs/fixed-form-test.f  2>&1 | FileCheck %s --check-prefix=MULTIPLEFORMS
+
+!-------------------------------------
+! EXPECTED OUTPUT FOR A FREE FORM FILE
+!-------------------------------------
+! FREEFORM:program a
+! FREEFORM-NOT:programa
+
+!---------------------------------------
+! EXPECTED OUTPUT FOR A FIXED FORM FILE
+!---------------------------------------
+! FIXEDFORM:programa
+! FIXEDFORM-NOT:program a
+
+!------------------------------------------------
+! EXPECTED OUTPUT FOR 2 FILES OF DIFFERENT FORMS
+!------------------------------------------------
+! MULTIPLEFORMS:program a
+! MULTIPLEFORMS-NOT:programa
+! MULTIPLEFORMS-NEXT:end
+! MULTIPLEFORMS-NEXT:programa
+! MULTIPLEFORMS-NOT:program a
\ No newline at end of file