diff --git a/cmake/HandleEigen.cmake b/cmake/HandleEigen.cmake
index b21d16885..c49eb4f8e 100644
--- a/cmake/HandleEigen.cmake
+++ b/cmake/HandleEigen.cmake
@@ -1,7 +1,10 @@
 ###############################################################################
 # Option for using system Eigen or GTSAM-bundled Eigen
-
-option(GTSAM_USE_SYSTEM_EIGEN "Find and use system-installed Eigen. If 'off', use the one bundled with GTSAM" OFF)
+# Default: Use system's Eigen if found automatically:
+find_package(Eigen3 QUIET)
+set(USE_SYSTEM_EIGEN_INITIAL_VALUE ${Eigen3_FOUND})
+option(GTSAM_USE_SYSTEM_EIGEN "Find and use system-installed Eigen. If 'off', use the one bundled with GTSAM" ${USE_SYSTEM_EIGEN_INITIAL_VALUE})
+unset(USE_SYSTEM_EIGEN_INITIAL_VALUE)
 
 if(NOT GTSAM_USE_SYSTEM_EIGEN)
   # This option only makes sense if using the embedded copy of Eigen, it is
@@ -11,7 +14,7 @@ endif()
 
 # Switch for using system Eigen or GTSAM-bundled Eigen
 if(GTSAM_USE_SYSTEM_EIGEN)
-    find_package(Eigen3 REQUIRED)
+    find_package(Eigen3 REQUIRED) # need to find again as REQUIRED
 
     # Use generic Eigen include paths e.g. <Eigen/Core>
     set(GTSAM_EIGEN_INCLUDE_FOR_INSTALL "${EIGEN3_INCLUDE_DIR}")