The move comes as U.S. health officials said earlier this week Americans fully vaccinated against COVID-19 should go back to wearing masks in indoor public places in regions where the coronavirus is spreading rapidly.
All market, regional and divisional employees who work in multiple facilities and all campus office workers have to be vaccinated by Oct 4, the retailer said in a memo.
The world's largest retailer had said in May fully vaccinated employees could work without masks.
Grocer Kroger Co also said on Friday it would "strongly encourage" all customers and employees, including those who are vaccinated, to wear a mask when in its stores and facilities to stop the spread of the Delta variant.
Walmart retail workers will post signage at its stores to encourage customers to wear masks, according to a separate company memo. Retail workers would also receive an incentive of $150, double the amount it had been paying, to get inoculated, with those already paid $75 set to receive the rest next month.
Store managers should regularly check the U.S. Centers for Disease Control and Prevention's website for potential changes to mask guidance in different locations, the company said.
Bloomberg News reported earlier this week Apple Inc's plans to restore its mask requirement policy at most of its U.S. retail stores, even for vaccinated customers and staff.
(Reporting by Praveen Paramasivam and Aditi Sebastian in Bengaluru; Editing by Krishna Chandra Eluri and Maju Samuel)