In its latest bid to boost productivity and speed delivery, Amazon.com Inc. is deploying a robot army.

The Seattle online retailer has outfitted several U.S. warehouses with squat, orange, wheeled robots that move stocked shelves to workers, instead of having employees seek items amid long aisles of merchandise, according to people familiar with the matter. At a 1.2-million-square-foot warehouse in Tracy, Calif., about 60 miles east of San Francisco, Amazon AMZN, -1.85% this summer replaced four floors of fixed shelving with the robots, the people said.