diff --git a/cmake/GitCheckout.cmake b/cmake/GitCheckout.cmake
index 9201c33..4969aa3 100644
--- a/cmake/GitCheckout.cmake
+++ b/cmake/GitCheckout.cmake
@@ -24,16 +24,38 @@ endmacro()
 #
 # Arguments:
 #   - URL: The URL of the remote Git repository.
+#
+# Optional arguments:
+#   - REF: The reference (branch, tag, or commit) to check out the Git repository.
 function(git_checkout URL)
-  cmake_parse_arguments(ARG "" "ERROR_VARIABLE" "" ${ARGN})
+  cmake_parse_arguments(ARG "" "REF;ERROR_VARIABLE" "" ${ARGN})
+
+  # Clones the Git repository.
   execute_process(
     COMMAND git clone ${URL}
     RESULT_VARIABLE RES
   )
   if(NOT RES EQUAL 0)
     _set_error(
-      "Failed to clone ${URL} (${RES})"
+      "Failed to clone '${URL}' (${RES})"
       ERROR_VARIABLE ${ARG_ERROR_VARIABLE}
     )
   endif()
+
+  # Determines the directory of the cloned Git repository.
+  string(REGEX REPLACE ".*/" "" GIT_DIR ${URL})
+
+  if(ARG_REF)
+    # Checks out the Git repository to a specific reference.
+    execute_process(
+      COMMAND git -C ${GIT_DIR} checkout ${ARG_REF}
+      RESULT_VARIABLE RES
+    )
+    if(NOT RES EQUAL 0)
+      _set_error(
+        "Failed to check out '${GIT_DIR}' to '${ARG_REF}' (${RES})"
+        ERROR_VARIABLE ${ARG_ERROR_VARIABLE}
+      )
+    endif()
+  endif()
 endfunction()
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index e8cc45d..e19905a 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -14,6 +14,8 @@ add_cmake_test(
   GitCheckoutTest.cmake
   "Check out a Git repository"
   "Check out an invalid Git repository"
+  "Check out a Git repository on a specific ref"
+  "Check out a Git repository on a specific invalid ref"
 )
 
 add_cmake_test(
diff --git a/test/GitCheckoutTest.cmake b/test/GitCheckoutTest.cmake
index c278982..b46218d 100644
--- a/test/GitCheckoutTest.cmake
+++ b/test/GitCheckoutTest.cmake
@@ -29,7 +29,51 @@ if("Check out an invalid Git repository" MATCHES ${TEST_MATCHES})
     ERROR_VARIABLE ERR
   )
 
-  set(EXPECTED_ERR "Failed to clone https://github.com/threeal/invalid-project (128)")
+  set(EXPECTED_ERR "Failed to clone 'https://github.com/threeal/invalid-project' (128)")
+  if(NOT ${ERR} STREQUAL EXPECTED_ERR)
+    message(FATAL_ERROR "It should fail to check out because of '${EXPECTED_ERR}' but instead got '${ERR}'")
+  endif()
+endif()
+
+if("Check out a Git repository on a specific ref" MATCHES ${TEST_MATCHES})
+  math(EXPR TEST_COUNT "${TEST_COUNT} + 1")
+
+  if(EXISTS project-starter)
+    file(REMOVE_RECURSE project-starter)
+  endif()
+
+  include(GitCheckout)
+  git_checkout(https://github.com/threeal/project-starter REF 5a80d20)
+
+  if(NOT EXISTS project-starter)
+    message(FATAL_ERROR "The 'project-starter' directory should exist")
+  endif()
+
+  execute_process(
+    COMMAND git -C project-starter rev-parse --short HEAD
+    OUTPUT_VARIABLE COMMIT_SHA
+  )
+  string(STRIP ${COMMIT_SHA} COMMIT_SHA)
+  if(NOT COMMIT_SHA STREQUAL 5a80d20)
+    message(FATAL_ERROR "The commit SHA should be '5a80d20' but instead got '${COMMIT_SHA}'")
+  endif()
+endif()
+
+if("Check out a Git repository on a specific invalid ref" MATCHES ${TEST_MATCHES})
+  math(EXPR TEST_COUNT "${TEST_COUNT} + 1")
+
+  if(EXISTS project-starter)
+    file(REMOVE_RECURSE project-starter)
+  endif()
+
+  include(GitCheckout)
+  git_checkout(
+    https://github.com/threeal/project-starter
+    REF invalid-ref
+    ERROR_VARIABLE ERR
+  )
+
+  set(EXPECTED_ERR "Failed to check out 'project-starter' to 'invalid-ref' (1)")
   if(NOT ${ERR} STREQUAL EXPECTED_ERR)
     message(FATAL_ERROR "It should fail to check out because of '${EXPECTED_ERR}' but instead got '${ERR}'")
   endif()