diff --git a/flang/include/flang/FrontendTool/Utils.h b/flang/include/flang/FrontendTool/Utils.h
--- a/flang/include/flang/FrontendTool/Utils.h
+++ b/flang/include/flang/FrontendTool/Utils.h
@@ -14,6 +14,8 @@
 #ifndef LLVM_FLANG_FRONTENDTOOL_UTILS_H
 #define LLVM_FLANG_FRONTENDTOOL_UTILS_H
 
+#include "llvm/ADT/StringRef.h"
+
 namespace Fortran::frontend {
 
 class CompilerInstance;
@@ -31,6 +33,14 @@
 /// \return - True on success.
 bool ExecuteCompilerInvocation(CompilerInstance *flang);
 
+/// \param suffix The file extension
+/// \return True if the file extension should be processed as fixed form
+bool isFixedFormSuffix(llvm::StringRef suffix);
+
+/// \param suffix The file extension
+/// \return True if the file extension should be processed as free form
+bool isFreeFormSuffix(llvm::StringRef suffix);
+
 } // end namespace Fortran::frontend
 
 #endif // LLVM_FLANG_FRONTENDTOOL_UTILS_H
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
@@ -9,6 +9,7 @@
 #include "flang/Frontend/FrontendAction.h"
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/FrontendActions.h"
+#include "flang/FrontendTool/Utils.h"
 #include "llvm/Support/Errc.h"
 
 using namespace Fortran::frontend;
@@ -51,6 +52,12 @@
 
   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 = isFixedFormSuffix(pathSuffix);
+  }
 
   // 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
@@ -7,15 +7,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "flang/Frontend/FrontendOptions.h"
-#include "llvm/ADT/StringSwitch.h"
+#include "flang/FrontendTool/Utils.h"
 
 using namespace Fortran::frontend;
 
 InputKind FrontendOptions::GetInputKindForExtension(llvm::StringRef extension) {
-  return llvm::StringSwitch<InputKind>(extension)
-      // TODO: Should match the list in flang/test/lit.cfg.py
-      // FIXME: Currently this API allows at most 9 items per case.
-      .Cases("f", "F", "f77", "f90", "F90", "f95", "F95", "ff95", "f18", "F18",
-          Language::Fortran)
-      .Default(Language::Unknown);
+  if (isFixedFormSuffix(extension) || isFreeFormSuffix(extension)) {
+    return Language::Fortran;
+  }
+  return Language::Unknown;
 }
diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -84,4 +84,19 @@
   return success;
 }
 
+bool isFixedFormSuffix(llvm::StringRef suffix) {
+  // Note: Keep this list in-sync with flang/test/lit.cfg.py
+  return suffix == "f" || suffix == "F" || suffix == "ff" || suffix == "for" ||
+      suffix == "FOR" || suffix == "fpp" || suffix == "FPP";
+}
+
+bool isFreeFormSuffix(llvm::StringRef suffix) {
+  // Note: Keep this list in-sync with flang/test/lit.cfg.py
+  // TODO: Add Cuda Fortan files (i.e. `*.cuf` and `*.CUF`).
+  return suffix == "f77" || suffix == "f90" || suffix == "F90" ||
+      suffix == "ff90" || suffix == "f95" || suffix == "F95" ||
+      suffix == "ff95" || suffix == "f03" || suffix == "F03" ||
+      suffix == "f08" || suffix == "F08" || suffix == "f18" || suffix == "F18";
+}
+
 } // namespace Fortran::frontend
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 FixedForm
+      end
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 FreeForm
+end
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 +1,40 @@
+! Ensure the driver correctly switches between fixed and free form based on the file extension.
+! This test exploits the fact that the preprocessor treats white-spaces differently for free
+! and fixed form input files.
+
+! 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 freeform
+! FREEFORM-NOT:programfixedform
+
+!---------------------------------------
+! EXPECTED OUTPUT FOR A FIXED FORM FILE
+!---------------------------------------
+! FIXEDFORM:programfixedform
+! FIXEDFORM-NOT:program freeform
+
+!------------------------------------------------
+! EXPECTED OUTPUT FOR 2 FILES OF DIFFERENT FORMS
+!------------------------------------------------
+! MULTIPLEFORMS:program freeform
+! MULTIPLEFORMS-NOT:programfixedform
+! MULTIPLEFORMS-NEXT:end
+! MULTIPLEFORMS-NEXT:programfixedform
+! MULTIPLEFORMS-NOT:program freeform
diff --git a/flang/test/lit.cfg.py b/flang/test/lit.cfg.py
--- a/flang/test/lit.cfg.py
+++ b/flang/test/lit.cfg.py
@@ -26,8 +26,8 @@
 
 # suffixes: A list of file extensions to treat as test files.
 config.suffixes = ['.c', '.cpp', '.f', '.F', '.ff', '.FOR', '.for', '.f77', '.f90', '.F90',
-                   '.ff90', '.f95', '.F95', '.ff95', '.fpp', '.FPP', '.cuf',
-                   '.CUF', '.f18', '.F18', '.fir']
+                   '.ff90', '.f95', '.F95', '.ff95', '.fpp', '.FPP', '.cuf'
+                   '.CUF', '.f18', '.F18', '.fir', '.f03', '.F03', '.f08', '.F08']
 
 config.substitutions.append(('%PATH%', config.environment['PATH']))
 
diff --git a/flang/unittests/Frontend/FrontendActionTest.cpp b/flang/unittests/Frontend/FrontendActionTest.cpp
--- a/flang/unittests/Frontend/FrontendActionTest.cpp
+++ b/flang/unittests/Frontend/FrontendActionTest.cpp
@@ -40,7 +40,7 @@
     // Generate a unique test file name.
     const testing::TestInfo *const test_info =
         testing::UnitTest::GetInstance()->current_test_info();
-    inputFileName_ = std::string(test_info->name()) + "_test-file.f";
+    inputFileName_ = std::string(test_info->name()) + "_test-file.f90";
 
     // Create the input file stream. Note that this stream is populated
     // separately in every test (i.e. the input is test specific).