Spongospora subterranea (Ss) is a soil-borne pathogen that infects multiple plant species and causes powdery scab disease on potato. Recently, growers in multiple states reported powdery scab symptoms on tubers grown in peat-based potting mix considered to be free of potato pathogens. Peat-based potting mix is commonly used to grow ornamental plants and in the production of seed potatoes in greenhouses. We obtained two unused commercial potting mix samples from growers who observed powdery scab on their tubers and tested them to determine if potting mix was the source of Ss. The number of Ss sporosori (resting spore aggregates) was determined with a validated qPCR assay. The number of sporosori per gram of potting mix (sp/g) ranged from 0 to 140 sp/g or 0 to 78 sp/g, making it difficult to determine whether the samples contained Ss. For confirmation, two bioassays were performed, using tomato plants as bait. For each assay, six tomato plants were grown in potting mix samples, in potting mix inoculated with 9 sp/g of Ss (positive control), and in autoclaved potting mix (negative control). Plasmodia were observed in the roots of plants grown in the potting mix samples and the positive control, but not in the negative control. The presence of Ss in tomato bait roots was confirmed using qPCR. These data provide evidence that Ss can be found in commercial potting mix. Our findings suggest that potting mix could contribute to the spread of Ss in the U.S. and could impact research and production of potatoes and other plants.