To examine the impacts of Seattle’s minimum wage ordinance on food prices by food processing category.

Supermarket food prices were collected for 106 items using a University of Washington Center for Public Health Nutrition market basket at affected and unaffected supermarket chain stores at three times: March 2015 (1-month pre-policy enactment), May 2015 (1-month post-policy enactment) and May 2016 (1-year post-policy enactment). Food items were categorized into four food processing groups, from minimally to ultra-processed. Data were analysed across time using a multilevel, linear difference-in-differences model at the store and price level stratified by level of food processing.

Six large supermarket chain stores located in Seattle (‘intervention’) affected by the policy and six same-chain but unaffected stores in King County (‘control’), Washington, USA.

One hundred and six food and beverage items.

The largest change in average price by food item was +$US 0·53 for ‘processed foods’ in King County between 1-month post-policy and 1-year post-policy enactment (P < 0·01). The smallest change was $US 0·00 for ‘unprocessed or minimally processed foods’ in Seattle between 1-month post-policy and 1-year post-policy enactment (P = 0·94). No significant changes in averaged chain prices were observed across food processing level strata in Seattle v. King County stores at 1-month or 1-year post-policy enactment.

Supermarket food prices do not appear to be differentially impacted by Seattle’s minimum wage ordinance by level of the food’s processing. These results suggest that the early implementation of a city-level minimum wage policy does not alter supermarket food prices by level of food processing.