diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index 2e27bc2279ac4..e6de8df5cef15 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -247,6 +247,8 @@ else()
   include_directories(SYSTEM ${MLIR_TABLEGEN_OUTPUT_DIR})
 endif()
 
+option(FLANG_INCLUDE_RUNTIME "Build the runtime in-tree (deprecated; to be replaced with LLVM_ENABLE_RUNTIMES=flang-rt)" ON)
+
 set(FLANG_TOOLS_INSTALL_DIR "${CMAKE_INSTALL_BINDIR}" CACHE PATH
     "Path for binary subdirectory (defaults to '${CMAKE_INSTALL_BINDIR}')")
 mark_as_advanced(FLANG_TOOLS_INSTALL_DIR)
@@ -487,7 +489,9 @@ if (FLANG_BUILD_TOOLS)
   add_subdirectory(tools)
 endif()
 
-add_subdirectory(runtime)
+if (FLANG_INCLUDE_RUNTIME)
+  add_subdirectory(runtime)
+endif ()
 
 if (LLVM_INCLUDE_EXAMPLES)
   add_subdirectory(examples)
diff --git a/flang/examples/CMakeLists.txt b/flang/examples/CMakeLists.txt
index 8cc66ddbbbb0e..23c6e790791fb 100644
--- a/flang/examples/CMakeLists.txt
+++ b/flang/examples/CMakeLists.txt
@@ -1,4 +1,6 @@
-add_subdirectory(ExternalHelloWorld)
+if (FLANG_INCLUDE_RUNTIME)
+  add_subdirectory(ExternalHelloWorld)
+endif ()
 add_subdirectory(PrintFlangFunctionNames)
 add_subdirectory(FlangOmpReport)
 add_subdirectory(FeatureList)
diff --git a/flang/test/CMakeLists.txt b/flang/test/CMakeLists.txt
index cab214c2ef4c8..e398e0786147a 100644
--- a/flang/test/CMakeLists.txt
+++ b/flang/test/CMakeLists.txt
@@ -71,9 +71,13 @@ set(FLANG_TEST_DEPENDS
   llvm-objdump
   llvm-readobj
   split-file
-  FortranRuntime
   FortranDecimal
 )
+
+if (FLANG_INCLUDE_RUNTIME)
+  list(APPEND FLANG_TEST_DEPENDS FortranRuntime)
+endif ()
+
 if (LLVM_ENABLE_PLUGINS AND NOT WIN32)
   list(APPEND FLANG_TEST_DEPENDS Bye)
 endif()
diff --git a/flang/test/Driver/ctofortran.f90 b/flang/test/Driver/ctofortran.f90
index 78eac32133b18..10c7adaccc958 100644
--- a/flang/test/Driver/ctofortran.f90
+++ b/flang/test/Driver/ctofortran.f90
@@ -1,4 +1,5 @@
 ! UNSUPPORTED: system-windows
+! REQUIRES: flang-rt
 ! RUN: split-file %s %t
 ! RUN: chmod +x %t/runtest.sh
 ! RUN: %t/runtest.sh %t %t/ffile.f90 %t/cfile.c %flang | FileCheck %s
diff --git a/flang/test/Driver/exec.f90 b/flang/test/Driver/exec.f90
index fd174005ddf62..9ca91ee24011c 100644
--- a/flang/test/Driver/exec.f90
+++ b/flang/test/Driver/exec.f90
@@ -1,4 +1,5 @@
 ! UNSUPPORTED: system-windows
+! REQUIRES: flang-rt
 ! Verify that flang can correctly build executables.
 
 ! RUN: %flang %s -o %t
diff --git a/flang/test/Runtime/no-cpp-dep.c b/flang/test/Runtime/no-cpp-dep.c
index b1a5fa004014c..7303ce63fdec4 100644
--- a/flang/test/Runtime/no-cpp-dep.c
+++ b/flang/test/Runtime/no-cpp-dep.c
@@ -3,7 +3,7 @@ This test makes sure that flang's runtime does not depend on the C++ runtime
 library. It tries to link this simple file against libFortranRuntime.a with
 a C compiler.
 
-REQUIRES: c-compiler
+REQUIRES: c-compiler, flang-rt
 
 RUN: %if system-aix %{ export OBJECT_MODE=64 %}
 RUN: %cc -std=c99 %s -I%include %libruntime -lm  \
diff --git a/flang/test/lit.cfg.py b/flang/test/lit.cfg.py
index c452b6d231c89..f4580afc8c47b 100644
--- a/flang/test/lit.cfg.py
+++ b/flang/test/lit.cfg.py
@@ -163,10 +163,13 @@
         ToolSubst("%not_todo_abort_cmd", command=FindTool("not"), unresolved="fatal")
     )
 
+if config.flang_include_runtime:
+    config.available_features.add("flang-rt")
+
 # Define some variables to help us test that the flang runtime doesn't depend on
 # the C++ runtime libraries. For this we need a C compiler. If for some reason
 # we don't have one, we can just disable the test.
-if config.cc:
+if config.flang_include_runtime and config.cc:
     libruntime = os.path.join(config.flang_lib_dir, "libFortranRuntime.a")
     include = os.path.join(config.flang_src_dir, "include")
 
diff --git a/flang/test/lit.site.cfg.py.in b/flang/test/lit.site.cfg.py.in
index d1a0ac763cf8a..697ba3fa79763 100644
--- a/flang/test/lit.site.cfg.py.in
+++ b/flang/test/lit.site.cfg.py.in
@@ -1,6 +1,7 @@
 @LIT_SITE_CFG_IN_HEADER@
 
 import sys
+import lit.util
 
 config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
 config.llvm_shlib_dir = lit_config.substitute(path(r"@SHLIBDIR@"))
@@ -32,6 +33,7 @@ else:
     config.openmp_module_dir = None
 config.flang_runtime_f128_math_lib = "@FLANG_RUNTIME_F128_MATH_LIB@"
 config.have_ldbl_mant_dig_113 = "@HAVE_LDBL_MANT_DIG_113@"
+config.flang_include_runtime = lit.util.pythonize_bool("@FLANG_INCLUDE_RUNTIME@")
 
 import lit.llvm
 lit.llvm.initialize(lit_config, config)
diff --git a/flang/tools/f18/CMakeLists.txt b/flang/tools/f18/CMakeLists.txt
index cc2bc5b8eb5ce..85ba2c74cdeb5 100644
--- a/flang/tools/f18/CMakeLists.txt
+++ b/flang/tools/f18/CMakeLists.txt
@@ -109,7 +109,7 @@ if (NOT CMAKE_CROSSCOMPILING)
     set(compile_with "-fsyntax-only")
     set(object_output "")
     set(include_in_link FALSE)
-    if(${filename} IN_LIST MODULES_WITH_IMPLEMENTATION)
+    if(${filename} IN_LIST MODULES_WITH_IMPLEMENTATION AND FLANG_INCLUDE_RUNTIME)
       set(object_output "${CMAKE_CURRENT_BINARY_DIR}/${filename}${CMAKE_CXX_OUTPUT_EXTENSION}")
       set(compile_with -c -o ${object_output})
       set(include_in_link TRUE)
diff --git a/flang/unittests/CMakeLists.txt b/flang/unittests/CMakeLists.txt
index 945067fed4f82..ecb7d68d8f729 100644
--- a/flang/unittests/CMakeLists.txt
+++ b/flang/unittests/CMakeLists.txt
@@ -1,3 +1,5 @@
+include(AddFlangOffloadRuntime)
+
 if (FLANG_EXPERIMENTAL_CUDA_RUNTIME)
   # If Fortran runtime is built as CUDA library, the linking
   # of targets that link FortranRuntime must be done
@@ -11,6 +13,11 @@ add_custom_target(FlangUnitTests)
 set_target_properties(FlangUnitTests PROPERTIES FOLDER "Flang/Tests")
 
 function(add_flang_unittest_offload_properties target)
+  # Do not apply runtime properties if not even compiling the runtime.
+  if (NOT FLANG_INCLUDE_RUNTIME)
+    return ()
+  endif ()
+
   # Set CUDA_RESOLVE_DEVICE_SYMBOLS.
   if (FLANG_EXPERIMENTAL_CUDA_RUNTIME)
     set_target_properties(${target}
@@ -75,5 +82,7 @@ add_subdirectory(Optimizer)
 add_subdirectory(Common)
 add_subdirectory(Decimal)
 add_subdirectory(Evaluate)
-add_subdirectory(Runtime)
+if (FLANG_INCLUDE_RUNTIME)
+  add_subdirectory(Runtime)
+endif ()
 add_subdirectory(Frontend)
diff --git a/flang/unittests/Evaluate/CMakeLists.txt b/flang/unittests/Evaluate/CMakeLists.txt
index 8111ecd72cfc7..1c3fac29cd298 100644
--- a/flang/unittests/Evaluate/CMakeLists.txt
+++ b/flang/unittests/Evaluate/CMakeLists.txt
@@ -33,7 +33,6 @@ add_flang_nongtest_unittest(intrinsics
   FortranDecimal
   FortranSemantics
   FortranParser
-  FortranRuntime
 )
 
 add_flang_nongtest_unittest(logical
@@ -56,19 +55,21 @@ add_flang_nongtest_unittest(real
 )
 llvm_update_compile_flags(real.test)
 
-add_flang_nongtest_unittest(reshape
-  NonGTestTesting
-  FortranSemantics
-  FortranEvaluate
-  FortranRuntime
-)
+if (FLANG_INCLUDE_RUNTIME)
+  add_flang_nongtest_unittest(reshape
+    NonGTestTesting
+    FortranSemantics
+    FortranEvaluate
+    FortranRuntime
+  )
 
-add_flang_nongtest_unittest(ISO-Fortran-binding
-  NonGTestTesting
-  FortranEvaluate
-  FortranSemantics
-  FortranRuntime
-)
+  add_flang_nongtest_unittest(ISO-Fortran-binding
+    NonGTestTesting
+    FortranEvaluate
+    FortranSemantics
+    FortranRuntime
+  )
+endif ()
 
 add_flang_nongtest_unittest(folding
   FortranSupport