diff --git a/src/librustc_trans/back/write.rs b/src/librustc_trans/back/write.rs
index b717254ef0d25..40a69721495b8 100644
--- a/src/librustc_trans/back/write.rs
+++ b/src/librustc_trans/back/write.rs
@@ -315,11 +315,15 @@ impl ModuleConfig {
         // Copy what clang does by turning on loop vectorization at O2 and
         // slp vectorization at O3. Otherwise configure other optimization aspects
         // of this pass manager builder.
+        // Turn off vectorization for emscripten, as it's not very well supported.
         self.vectorize_loop = !sess.opts.cg.no_vectorize_loops &&
                              (sess.opts.optimize == config::OptLevel::Default ||
-                              sess.opts.optimize == config::OptLevel::Aggressive);
+                              sess.opts.optimize == config::OptLevel::Aggressive) &&
+                             !sess.target.target.options.is_like_emscripten;
+
         self.vectorize_slp = !sess.opts.cg.no_vectorize_slp &&
-                            sess.opts.optimize == config::OptLevel::Aggressive;
+                            sess.opts.optimize == config::OptLevel::Aggressive &&
+                            !sess.target.target.options.is_like_emscripten;
 
         self.merge_functions = sess.opts.optimize == config::OptLevel::Default ||
                                sess.opts.optimize == config::OptLevel::Aggressive;