Stepp is turning his attention to his family's winery in the German region of Pfalz, but will continue to work for M&S as a consultant.
Wine department manager Andrew Bird described Stepp as "a pillar of the M&S wine department for the past nine years".
He said: "I have no doubt that without Gerd's calm, efficient and uncompromising approach to quality, the M&S wine range would not have gained the numerous awards and enviable profile of recent years."
In the retailer's spring tasting brochure, Bird urged the trade to be on the look-out for a candidate to fill Stepp's shoes.
The tasting of M&S's new spring range revealed the supermarket is expanding its Argentinian range, with a host of new suppliers, following two trips to the country last year. Eight new wines will launch this spring .