Olympic mudminnow (Novumbra hubbsi) is a small species endemic to western Washington State that inhabits marshy, wetland-type areas with abundant aquatic macrophytes. Because they can be locally abundant and may be the only fish species present, they may have important effects on the aquatic community. However, little is known about their diet under natural conditions. To get a detailed account of their feeding ecology, Olympic mudminnow were sampled from six sites. Three sites were sampled monthly from February to August 2012 and the other three sites were sampled once in June 2012. Gastric lavage was used to collect stomach samples from fish ≥ 40 mm total length (TL). Stomach samples were collected from 477 fish (40–75 mm TL). Olympic mudminnow diet was comprised primarily of aquatic invertebrates. By weight, oligochaetes and chironomid larvae were the predominant prey types but by number, copepods were the predominant prey type. Of the three monthly sample sites, diet overlap between seasons was high at one site, low at another site, and variable at the third site. Diet overlap was generally low between sites. Olympic mudminnow 40–49 mm TL primarily displayed a generalist feeding strategy, whereas larger fish displayed more specialization. A consistent pattern of ontogenetic diet shifts was not apparent. Results of this study and an earlier study indicate Olympic mudminnow consume a wide variety of types and sizes of aquatic invertebrates and their diet can vary widely between sites and seasons.