The milestone has been reached in just two years, with Nisa converting an average of four stores a week.
The Store of the Future format includes a white fascia, contemporary window graphics and new internal furnishings and layout.
Manny Hussain's store in Motherwell, North Lanarkshire, was the 400th to be converted. The store is a new build and replaces a former unit that was destroyed by a fire last year.
"Everything in the store is new, from the walls to the floor, from chillers to freezers. We're pleased with the look and feel," he said.
Nisa-Today's group symbol director John Heagney added: "The positive feedback we're receiving from our members and their customers proves it's worth it. The symbol group is continually strengthening and we are working with members to deliver an outstanding service."