diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5d29839..c512ad62 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,6 +5,9 @@
     project("mbed TLS" C)
 endif()
 
+# Set the project root directory.
+set(MBEDTLS_DIR ${CMAKE_CURRENT_SOURCE_DIR})
+
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)
 
 option(UNSAFE_BUILD "Allow unsafe builds. These builds ARE NOT SECURE." OFF)
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 61bc13d..6b2a850 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -2,6 +2,13 @@
 option(USE_SHARED_MBEDTLS_LIBRARY "Build mbed TLS shared library." OFF)
 option(LINK_WITH_PTHREAD "Explicitly link mbed TLS library to pthread." OFF)
 
+# Set the project root directory if it's not already defined, as may happen if
+# the library folder is included directly by a parent project, without
+# including the top level CMakeLists.txt.
+if(NOT DEFINED MBEDTLS_DIR)
+    set(MBEDTLS_DIR ${CMAKE_SOURCE_DIR})
+endif()
+
 set(src_crypto
     aes.c
     aesni.c
@@ -72,9 +79,9 @@
 if(USE_CRYPTO_SUBMODULE)
 set(src_crypto
     ${src_crypto}
-    ${CMAKE_SOURCE_DIR}/library/version.c
-    ${CMAKE_SOURCE_DIR}/library/version_features.c
-    ${CMAKE_SOURCE_DIR}/library/error.c
+    ${MBEDTLS_DIR}/library/version.c
+    ${MBEDTLS_DIR}/library/version_features.c
+    ${MBEDTLS_DIR}/library/error.c
 )
 else()
 set(src_crypto
@@ -133,8 +140,8 @@
     set_target_properties(${mbedcrypto_static_target} PROPERTIES OUTPUT_NAME mbedcrypto)
     target_link_libraries(${mbedcrypto_static_target} ${libs})
     target_include_directories(${mbedcrypto_static_target}
-        PUBLIC ${CMAKE_SOURCE_DIR}/include/
-        PUBLIC ${CMAKE_SOURCE_DIR}/crypto/include/)
+        PUBLIC ${MBEDTLS_DIR}/include/
+        PUBLIC ${MBEDTLS_DIR}/crypto/include/)
 
     install(TARGETS ${mbedcrypto_static_target}
             DESTINATION ${LIB_INSTALL_DIR}
@@ -146,8 +153,8 @@
     set_target_properties(mbedcrypto PROPERTIES VERSION 2.17.0 SOVERSION 3)
     target_link_libraries(mbedcrypto ${libs})
     target_include_directories(mbedcrypto
-        PUBLIC ${CMAKE_SOURCE_DIR}/include/
-        PUBLIC ${CMAKE_SOURCE_DIR}/crypto/include/)
+        PUBLIC ${MBEDTLS_DIR}/include/
+        PUBLIC ${MBEDTLS_DIR}/crypto/include/)
 
     install(TARGETS mbedcrypto
             DESTINATION ${LIB_INSTALL_DIR}
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 42d99d6..9dc1908 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -2,6 +2,13 @@
     mbedcrypto
 )
 
+# Set the project root directory if it's not already defined, as may happen if
+# the tests folder is included directly by a parent project, without including
+# the top level CMakeLists.txt.
+if(NOT DEFINED MBEDTLS_DIR)
+    set(MBEDTLS_DIR ${CMAKE_SOURCE_DIR})
+endif()
+
 find_package(Perl)
 if(NOT PERL_FOUND)
     message(FATAL_ERROR "Cannot build test suites without Perl")
@@ -43,9 +50,9 @@
     add_executable(${exe_name} test_suite_${data_name}.c)
     target_link_libraries(${exe_name} ${libs})
     target_include_directories(${exe_name}
-        PUBLIC ${CMAKE_SOURCE_DIR}/include/
-        PUBLIC ${CMAKE_SOURCE_DIR}/crypto/include/
-        PRIVATE ${CMAKE_SOURCE_DIR}/crypto/library/)
+        PUBLIC ${MBEDTLS_DIR}/include/
+        PUBLIC ${MBEDTLS_DIR}/crypto/include/
+        PRIVATE ${MBEDTLS_DIR}/crypto/library/)
 
     if(${data_name} MATCHES ${SKIP_TEST_SUITES_REGEX})
         message(STATUS "The test suite ${data_name} will not be executed.")