Walmart-Backed Fintech Uses Retailer’s Reach to Challenge Banks

Walmart Inc. is one of the last remaining merchants that’s been able to hold out against the swell of Apple Pay in recent years. Now, it’s pushing a fledgling alternative to its huge trove of customers and employees.

Although it still has little name recognition among most Americans, a firm backed by Walmart — OnePay — is now growing rapidly. The financial-technology service has 6 million monthly active users, double the number a year ago, according to people with knowledge of the company’s results, who asked not to be identified discussing information that isn’t public. And OnePay has $50 billion ...