diff --git a/flang/lib/Parser/preprocessor.cpp b/flang/lib/Parser/preprocessor.cpp
index 7d3130cd66ed99..cb3725bc4ea67e 100644
--- a/flang/lib/Parser/preprocessor.cpp
+++ b/flang/lib/Parser/preprocessor.cpp
@@ -769,7 +769,7 @@ void Preprocessor::Directive(const TokenSequence &dir, Prescanner &prescanner) {
       if (included->bytes() > 0) {
         ProvenanceRange fileRange{
             allSources_.AddIncludedFile(*included, dir.GetProvenanceRange())};
-        Prescanner{prescanner, /*isNestedInIncludeDirective=*/true}
+        Prescanner{prescanner, *this, /*isNestedInIncludeDirective=*/true}
             .set_encoding(included->encoding())
             .Prescan(fileRange);
       }
diff --git a/flang/lib/Parser/prescan.cpp b/flang/lib/Parser/prescan.cpp
index 7dcb61ac79109f..b594df8ecb6858 100644
--- a/flang/lib/Parser/prescan.cpp
+++ b/flang/lib/Parser/prescan.cpp
@@ -32,10 +32,10 @@ Prescanner::Prescanner(Messages &messages, CookedSource &cooked,
       backslashFreeFormContinuation_{preprocessor.AnyDefinitions()},
       encoding_{allSources_.encoding()} {}
 
-Prescanner::Prescanner(const Prescanner &that, bool isNestedInIncludeDirective)
-    : messages_{that.messages_}, cooked_{that.cooked_},
-      preprocessor_{that.preprocessor_}, allSources_{that.allSources_},
-      features_{that.features_},
+Prescanner::Prescanner(const Prescanner &that, Preprocessor &prepro,
+    bool isNestedInIncludeDirective)
+    : messages_{that.messages_}, cooked_{that.cooked_}, preprocessor_{prepro},
+      allSources_{that.allSources_}, features_{that.features_},
       isNestedInIncludeDirective_{isNestedInIncludeDirective},
       backslashFreeFormContinuation_{that.backslashFreeFormContinuation_},
       inFixedForm_{that.inFixedForm_},
@@ -1104,7 +1104,14 @@ void Prescanner::FortranInclude(const char *firstQuote) {
         provenance, static_cast<std::size_t>(p - nextLine_)};
     ProvenanceRange fileRange{
         allSources_.AddIncludedFile(*included, includeLineRange)};
-    Prescanner{*this, /*isNestedInIncludeDirective=*/false}
+    Preprocessor cleanPrepro{allSources_};
+    if (preprocessor_.IsNameDefined("__FILE__"s)) {
+      cleanPrepro.DefineStandardMacros(); // __FILE__, __LINE__, &c.
+    }
+    if (preprocessor_.IsNameDefined("_CUDA"s)) {
+      cleanPrepro.Define("_CUDA"s, "1");
+    }
+    Prescanner{*this, cleanPrepro, /*isNestedInIncludeDirective=*/false}
         .set_encoding(included->encoding())
         .Prescan(fileRange);
   }
diff --git a/flang/lib/Parser/prescan.h b/flang/lib/Parser/prescan.h
index a64df5377e7e03..9d4f7c0c302a1a 100644
--- a/flang/lib/Parser/prescan.h
+++ b/flang/lib/Parser/prescan.h
@@ -35,7 +35,8 @@ class Prescanner {
 public:
   Prescanner(Messages &, CookedSource &, Preprocessor &,
       common::LanguageFeatureControl);
-  Prescanner(const Prescanner &, bool isNestedInIncludeDirective);
+  Prescanner(
+      const Prescanner &, Preprocessor &, bool isNestedInIncludeDirective);
   Prescanner(const Prescanner &) = delete;
   Prescanner(Prescanner &&) = delete;
 
diff --git a/flang/module/__fortran_builtins.f90 b/flang/module/__fortran_builtins.f90
index a9d3ac897eb583..d1d4a639e70d72 100644
--- a/flang/module/__fortran_builtins.f90
+++ b/flang/module/__fortran_builtins.f90
@@ -6,7 +6,7 @@
 !
 !===------------------------------------------------------------------------===!
 
-include '../include/flang/Runtime/magic-numbers.h'
+#include '../include/flang/Runtime/magic-numbers.h'
 
 ! These naming shenanigans prevent names from Fortran intrinsic modules
 ! from being usable on INTRINSIC statements, and force the program
diff --git a/flang/module/__fortran_ieee_exceptions.f90 b/flang/module/__fortran_ieee_exceptions.f90
index cebd6045201812..6691012eda238a 100644
--- a/flang/module/__fortran_ieee_exceptions.f90
+++ b/flang/module/__fortran_ieee_exceptions.f90
@@ -11,7 +11,7 @@
 ! here under another name so that IEEE_ARITHMETIC can USE it and export its
 ! declarations without clashing with a non-intrinsic module in a program.
 
-include '../include/flang/Runtime/magic-numbers.h'
+#include '../include/flang/Runtime/magic-numbers.h'
 
 module __fortran_ieee_exceptions
   use __fortran_builtins, only: &
diff --git a/flang/module/ieee_arithmetic.f90 b/flang/module/ieee_arithmetic.f90
index 32e640b9e24574..7eaa7db55af9f8 100644
--- a/flang/module/ieee_arithmetic.f90
+++ b/flang/module/ieee_arithmetic.f90
@@ -8,7 +8,7 @@
 
 ! Fortran 2018 Clause 17
 
-include '../include/flang/Runtime/magic-numbers.h'
+#include '../include/flang/Runtime/magic-numbers.h'
 
 module ieee_arithmetic
   ! F18 Clause 17.1p1:
diff --git a/flang/module/iso_fortran_env.f90 b/flang/module/iso_fortran_env.f90
index cc1f58e7feccb5..4e575b422c2a04 100644
--- a/flang/module/iso_fortran_env.f90
+++ b/flang/module/iso_fortran_env.f90
@@ -8,7 +8,7 @@
 
 ! See Fortran 2023, subclause 16.10.2
 
-include '../include/flang/Runtime/magic-numbers.h'
+#include '../include/flang/Runtime/magic-numbers.h'
 
 module iso_fortran_env
 
diff --git a/flang/test/Driver/include-header.f90 b/flang/test/Driver/include-header.f90
index 789d99526d3620..28b75e072544e4 100644
--- a/flang/test/Driver/include-header.f90
+++ b/flang/test/Driver/include-header.f90
@@ -51,7 +51,7 @@ program B
 end
 
 ! include-test-two.f90
-INCLUDE "basic-header-two.h"
+#include "basic-header-two.h"
 #ifdef Y
 program Y
 #else
diff --git a/flang/test/Preprocessing/include-file.h b/flang/test/Preprocessing/include-file.h
new file mode 100644
index 00000000000000..4d4404e75e0e3b
--- /dev/null
+++ b/flang/test/Preprocessing/include-file.h
@@ -0,0 +1 @@
+print *, sin(0.), j
diff --git a/flang/test/Preprocessing/include-line.F90 b/flang/test/Preprocessing/include-line.F90
new file mode 100644
index 00000000000000..63ff9d33b07b40
--- /dev/null
+++ b/flang/test/Preprocessing/include-line.F90
@@ -0,0 +1,6 @@
+! RUN: %flang_fc1 -fdebug-unparse %s -Dj=1 2>&1 | FileCheck %s
+! Ensure that macro definitions don't affect INCLUDE lines (unlike #include)
+#define sin cos
+!CHECK: PRINT *, 0._4, j
+include "include-file.h"
+end