diff --git a/include/flatbuffers/flatbuffers.h b/include/flatbuffers/flatbuffers.h
index 0ae297b39..0b9cf5853 100644
--- a/include/flatbuffers/flatbuffers.h
+++ b/include/flatbuffers/flatbuffers.h
@@ -416,7 +416,10 @@ class vector_downward {
   uint8_t *make_space(size_t len) {
     if (len > static_cast<size_t>(cur_ - buf_)) {
       auto old_size = size();
+      auto largest_align = AlignOf<largest_scalar_t>();
       reserved_ += std::max(len, growth_policy(reserved_));
+      // Round up to avoid undefined behavior from unaligned loads and stores.
+      reserved_ = (reserved_ + (largest_align - 1)) & ~(largest_align - 1);
       auto new_buf = allocator_.allocate(reserved_);
       auto new_cur = new_buf + reserved_ - old_size;
       memcpy(new_cur, cur_, old_size);