Google and Wal-Mart Stores Inc. are joining forces in a partnership that includes enabling voice-ordered purchases from the retail giant on Google’s virtual assistant, challenging rival Amazon.com Inc.’s grip on the next wave of e-commerce.

Wal-Mart said Wednesday that next month it will join Google’s online-shopping marketplace, Google Express. While the deal will add hundreds of thousands of Wal-Mart items to Google Express, it will also give Wal-Mart access to voice ordering. The deal won’t alter how consumers receive their...