diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -230,7 +230,13 @@
   };
 
   for (size_t i = 0, n = nList.size(); i < n; ++i) {
-    const nlist_64 &sym = nList[i];
+    nlist_64 sym;
+    // nlist_64 structs should be 64-bit-aligned according to the C++ language
+    // spec, but nList (which is mmap'ed from a file) may not be at an aligned
+    // offset. This memcpy() ensures that we have the correct alignment, but
+    // it should be optimized away on architectures that support unaligned
+    // reads.
+    memcpy(&sym, nList.data() + i, sizeof(nlist_64));
 
     // Undefined symbol
     if (!sym.n_sect) {