diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index 05e21ef2d33b5..c66688b42a3b6 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -509,6 +509,8 @@ end
 * We respect Fortran comments in macro actual arguments (like GNU, Intel, NAG;
   unlike PGI and XLF) on the principle that macro calls should be treated
   like function references.  Fortran's line continuation methods also work.
+* We implement the `__COUNTER__` preprocessing extension,
+  see [Non-standard Extensions](Preprocessing#non-standard-extensions)
 
 ## Standard features not silently accepted
 
diff --git a/flang/docs/Preprocessing.md b/flang/docs/Preprocessing.md
index 0b70d857833ce..db815b9244edf 100644
--- a/flang/docs/Preprocessing.md
+++ b/flang/docs/Preprocessing.md
@@ -138,6 +138,18 @@ text.
 OpenMP-style directives that look like comments are not addressed by
 this scheme but are obvious extensions.
 
+## Currently implemented built-ins
+
+* `__DATE__`: Date, given as e.g. "Jun 16 1904"
+* `__TIME__`: Time in 24-hour format including seconds, e.g. "09:24:13"
+* `__TIMESTAMP__`: Date, time and year of last modification, given as e.g. "Fri May  9 09:16:17 2025"
+* `__FILE__`: Current file
+* `__LINE__`: Current line
+
+### Non-standard Extensions
+
+* `__COUNTER__`: Replaced by sequential integers on each expansion, starting from 0.
+
 ## Appendix
 `N` in the table below means "not supported"; this doesn't
 mean a bug, it just means that a particular behavior was
diff --git a/flang/include/flang/Parser/preprocessor.h b/flang/include/flang/Parser/preprocessor.h
index 86528a7e68def..834c84a639a74 100644
--- a/flang/include/flang/Parser/preprocessor.h
+++ b/flang/include/flang/Parser/preprocessor.h
@@ -121,6 +121,8 @@ class Preprocessor {
   std::list<std::string> names_;
   std::unordered_map<CharBlock, Definition> definitions_;
   std::stack<CanDeadElseAppear> ifStack_;
+
+  unsigned int counterVal_{0};
 };
 } // namespace Fortran::parser
 #endif // FORTRAN_PARSER_PREPROCESSOR_H_
diff --git a/flang/lib/Parser/preprocessor.cpp b/flang/lib/Parser/preprocessor.cpp
index a47f9c32ad27c..4549b1c505569 100644
--- a/flang/lib/Parser/preprocessor.cpp
+++ b/flang/lib/Parser/preprocessor.cpp
@@ -22,6 +22,7 @@
 #include <memory>
 #include <optional>
 #include <set>
+#include <string>
 #include <utility>
 #include <vector>
 
@@ -299,6 +300,7 @@ void Preprocessor::DefineStandardMacros() {
   Define("__FILE__"s, "__FILE__"s);
   Define("__LINE__"s, "__LINE__"s);
   Define("__TIMESTAMP__"s, "__TIMESTAMP__"s);
+  Define("__COUNTER__"s, "__COUNTER__"s);
 }
 
 void Preprocessor::Define(const std::string &macro, const std::string &value) {
@@ -421,6 +423,8 @@ std::optional<TokenSequence> Preprocessor::MacroReplacement(
               repl = "\""s + time + '"';
             }
           }
+        } else if (name == "__COUNTER__") {
+          repl = std::to_string(counterVal_++);
         }
         if (!repl.empty()) {
           ProvenanceRange insert{allSources_.AddCompilerInsertion(repl)};
diff --git a/flang/test/Preprocessing/counter.F90 b/flang/test/Preprocessing/counter.F90
new file mode 100644
index 0000000000000..9761c8fb7f355
--- /dev/null
+++ b/flang/test/Preprocessing/counter.F90
@@ -0,0 +1,9 @@
+! RUN: %flang -E %s | FileCheck %s
+! CHECK: print *, 0
+! CHECK: print *, 1
+! CHECK: print *, 2
+! Check incremental counter macro
+#define foo bar
+print *, __COUNTER__
+print *, __COUNTER__
+print *, __COUNTER__