diff --git a/.github/workflows/CI-unixish.yml b/.github/workflows/CI-unixish.yml
index 60361389..f76fdd15 100644
--- a/.github/workflows/CI-unixish.yml
+++ b/.github/workflows/CI-unixish.yml
@@ -73,6 +73,9 @@ jobs:
       run: |
         cmake --build cmake.output --target testrunner -- -j $(nproc)
         ./cmake.output/testrunner
+        # Re-run tests from within the build directory to validate that
+        # SIMPLECPP_TEST_SOURCE_DIR is correctly defined and resolved
+        (cd cmake.output && ./testrunner)
 
     - name: Run valgrind
       if: matrix.os == 'ubuntu-24.04'
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f9e3eb6d..8799b7a4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,6 +76,10 @@ add_library(simplecpp_obj OBJECT simplecpp.cpp)
 
 add_executable(simplecpp $<TARGET_OBJECTS:simplecpp_obj> main.cpp)
 add_executable(testrunner $<TARGET_OBJECTS:simplecpp_obj> test.cpp)
+target_compile_definitions(testrunner
+    PRIVATE
+        SIMPLECPP_TEST_SOURCE_DIR="${CMAKE_CURRENT_SOURCE_DIR}"
+)
 
 enable_testing()
 add_test(NAME testrunner COMMAND testrunner)
diff --git a/Makefile b/Makefile
index d899d2cd..7489ec83 100644
--- a/Makefile
+++ b/Makefile
@@ -1,11 +1,17 @@
 all:	testrunner simplecpp
 
+CPPFLAGS ?=
 CXXFLAGS = -Wall -Wextra -pedantic -Wcast-qual -Wfloat-equal -Wmissing-declarations -Wmissing-format-attribute -Wredundant-decls -Wundef -Wno-multichar -Wold-style-cast -std=c++11 -g $(CXXOPTS)
 LDFLAGS = -g $(LDOPTS)
 
-%.o: %.cpp	simplecpp.h
-	$(CXX) $(CXXFLAGS) -c $<
+# Define test source dir macro for compilation (preprocessor flags)
+TEST_CPPFLAGS = -DSIMPLECPP_TEST_SOURCE_DIR=\"$(CURDIR)\"
+
+# Only test.o gets the define
+test.o: CPPFLAGS += $(TEST_CPPFLAGS)
 
+%.o: %.cpp	simplecpp.h
+	$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $<
 
 testrunner:	test.o	simplecpp.o
 	$(CXX) $(LDFLAGS) simplecpp.o test.o -o testrunner
diff --git a/test.cpp b/test.cpp
index 7b108638..c44524a3 100644
--- a/test.cpp
+++ b/test.cpp
@@ -16,9 +16,14 @@
 #include <utility>
 #include <vector>
 
+#ifndef SIMPLECPP_TEST_SOURCE_DIR
+#error "SIMPLECPP_TEST_SOURCE_DIR is not defined."
+#endif
+
 #define STRINGIZE_(x) #x
 #define STRINGIZE(x) STRINGIZE_(x)
 
+static const std::string testSourceDir = SIMPLECPP_TEST_SOURCE_DIR;
 static int numberOfFailedAssertions = 0;
 
 #define ASSERT_EQUALS(expected, actual)  (assertEquals((expected), (actual), __LINE__))
@@ -1556,6 +1561,7 @@ static void has_include_1()
                         "  #endif\n"
                         "#endif";
     simplecpp::DUI dui;
+    dui.includePaths.push_back(testSourceDir);
     dui.std = "c++17";
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     dui.std = "c++14";
@@ -1573,6 +1579,7 @@ static void has_include_2()
                         "  #endif\n"
                         "#endif";
     simplecpp::DUI dui;
+    dui.includePaths.push_back(testSourceDir);
     dui.std = "c++17";
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     ASSERT_EQUALS("", preprocess(code));
@@ -1592,7 +1599,7 @@ static void has_include_3()
     // Test file not found...
     ASSERT_EQUALS("\n\n\n\nB", preprocess(code, dui));
     // Unless -I is set (preferably, we should differentiate -I and -isystem...)
-    dui.includePaths.push_back("./testsuite");
+    dui.includePaths.push_back(testSourceDir + "/testsuite");
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     ASSERT_EQUALS("", preprocess(code));
 }
@@ -1608,6 +1615,7 @@ static void has_include_4()
                         "#endif";
     simplecpp::DUI dui;
     dui.std = "c++17";
+    dui.includePaths.push_back(testSourceDir);
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     ASSERT_EQUALS("", preprocess(code));
 }
@@ -1623,6 +1631,7 @@ static void has_include_5()
                         "#endif";
     simplecpp::DUI dui;
     dui.std = "c++17";
+    dui.includePaths.push_back(testSourceDir);
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     ASSERT_EQUALS("", preprocess(code));
 }
@@ -1638,6 +1647,7 @@ static void has_include_6()
                         "#endif";
     simplecpp::DUI dui;
     dui.std = "gnu99";
+    dui.includePaths.push_back(testSourceDir);
     ASSERT_EQUALS("\n\nA", preprocess(code, dui));
     ASSERT_EQUALS("", preprocess(code));
 }