diff --git a/CMakeLists.txt b/CMakeLists.txt
index 09c9ec5358..09edf81f97 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -59,6 +59,32 @@ if(CHECK_PIE_SUPPORTED)
   endif()
 endif()
 
+include(CheckCSourceCompiles)
+
+check_c_source_compiles("
+#if __riscv_xlen == 64
+int main() { return 0; }
+#else
+#error Not RISCV-64
+#endif
+" IS_RISCV_XLEN_64)
+
+check_c_source_compiles("
+#if __riscv_xlen == 32
+int main() { return 0; }
+#else
+#error Not RISCV-32
+#endif
+" IS_RISCV_XLEN_32)
+
+if(IS_RISCV_XLEN_32)
+  set(RISCV_XLEN 32)
+elseif(IS_RISCV_XLEN_64)
+  set(RISCV_XLEN 64)
+else()
+  message(WARNING "Unable to determine RISC-V XLEN")
+endif()
+
 include(GNUInstallDirs)
 
 if (NOT CMAKE_BUILD_TYPE)
@@ -72,7 +98,7 @@ set(HWY_CMAKE_ARM7 OFF CACHE BOOL "Set copts for Armv7 with NEON (requires vfpv4
 # skipped. For GCC 13.1+, you can also build with -fexcess-precision=standard.
 set(HWY_CMAKE_SSE2 OFF CACHE BOOL "Set SSE2 as baseline for 32-bit x86?")
 
-# Currently this will compile the entire codebase with `-march=rv64gcv1p0`:
+# Currently this will compile the entire codebase with `-march=rv<XLEN>gcv1p0`:
 set(HWY_CMAKE_RVV ON CACHE BOOL "Set copts for RISCV with RVV?")
 
 # Unconditionally adding -Werror risks breaking the build when new warnings
@@ -378,8 +404,13 @@ else()
     # we add the gcv compiler flag, which then requires the CPU (now when using
     # either compiler) to support V.
     if(HWY_CMAKE_RVV)
-      list(APPEND HWY_FLAGS -march=rv64gcv1p0)
-      add_link_options(-march=rv64gcv1p0)
+      if(RISCV_XLEN EQUAL 64)
+        list(APPEND HWY_FLAGS -march=rv64gcv1p0)
+        add_link_options(-march=rv64gcv1p0)
+      elseif(RISCV_XLEN EQUAL 32)
+        list(APPEND HWY_FLAGS -march=rv32gcv1p0)
+        add_link_options(-march=rvgcv1p0)
+      endif()
       if(${CMAKE_CXX_COMPILER_ID} MATCHES "Clang")
         list(APPEND HWY_FLAGS -menable-experimental-extensions)
       endif()