diff --git a/ps2xRuntime/CMakeLists.txt b/ps2xRuntime/CMakeLists.txt
index e4dc195..aad5fb8 100644
--- a/ps2xRuntime/CMakeLists.txt
+++ b/ps2xRuntime/CMakeLists.txt
@@ -552,6 +552,11 @@ if(PS2X_IS_VITA)
     endif()
 endif()
 
+# Every configuration, not just Release: SSE4.1 is required to compile, not an
+# optimisation. PUBLIC so recompiled game code linking against ps2_runtime
+# inherits it -- that code is where most of the SSE4.1 intrinsics actually are.
+EnableX86SimdBaseline(ps2_runtime)
+
 if(CMAKE_BUILD_TYPE STREQUAL "Release" OR CMAKE_BUILD_TYPE STREQUAL "RelWithDebInfo")
     EnableFastReleaseMode(ps2_runtime)
     EnableFastReleaseMode(ps2EntryRunner)
diff --git a/ps2xRuntime/cmake/ReleaseMode.cmake b/ps2xRuntime/cmake/ReleaseMode.cmake
index 92ef0d5..0b97e90 100644
--- a/ps2xRuntime/cmake/ReleaseMode.cmake
+++ b/ps2xRuntime/cmake/ReleaseMode.cmake
@@ -2,6 +2,26 @@ include(CheckIPOSupported)
 
 check_ipo_supported(RESULT IPO_SUPPORTED OUTPUT IPO_ERROR)
 
+# ps2_runtime.h unconditionally includes <smmintrin.h> and the recompiler emits
+# SSE4.1-only intrinsics (_mm_blendv_ps and friends) for the COP2/FPU select
+# idioms, so SSE4.1 is a hard requirement of the codebase rather than a tuning
+# knob. MSVC enables it implicitly (its intrinsics are not gated by a target
+# feature), but GCC and Clang default to the plain x86-64 baseline, which is
+# SSE2 -- so on any stock Linux or macOS toolchain those translation units fail
+# to compile with "always_inline function ... requires target feature 'sse4.1'".
+#
+# This must not live in EnableFastReleaseMode: that is only applied for Release
+# and RelWithDebInfo, whereas the requirement applies to every configuration.
+function(EnableX86SimdBaseline TargetName)
+    if(MSVC)
+        return()
+    endif()
+    if(NOT CMAKE_SYSTEM_PROCESSOR MATCHES "^(x86_64|AMD64|amd64|i[3-6]86|x86)$")
+        return()
+    endif()
+    target_compile_options(${TargetName} PUBLIC -msse4.1)
+endfunction()
+
 function(EnableFastReleaseMode TargetName)
     message("> Enabling optimization for: ${TargetName}")
     if(MSVC)