Methane-derived carbon has been shown to support stoneflies in hyporheic zones and midges in lake sediments. But evidence for widespread aquatic food web reliance on these non-phytosynthetic energy pathways is limited. Here, we examined methane-derived carbon contributions to riverine food webs in southeast Alaska coastal watersheds. These rivers are highly oligotrophic, especially in winter when low light and temperatures limit photosynthesis. We conducted seasonal sampling in four streams to measure (1) stable isotopes (δ13C) of fish and aquatic invertebrate biomass, and (2) dissolved methane concentrations from surface and well-water. Winter methane concentrations were high in both surface and well water, ranging to 10 µM in the surface and 30 µM subsurface. Correspondingly, we found strong seasonal methane incorporation into river food webs evidenced by isotopic values. In the winter, aquatic invertebrates were highly depleted in 13C (δ13C of -65 to -45) relative to summer months (δ13C of -35 to -25), and isotope mixing model analyses suggested that methane was the primary energy source fueling the aquatic food web in winter. We combined this information with a previously constructed quantitative food web for one of our study sites to estimate the proportion of secondary invertebrate and fish production supported by methanotrophy. These seasonal methanotrophic pathways fueled an estimated 20% of annual secondary invertebrate production at this study site. Together, these findings suggest that methane could be a quantitatively significant energy source in these coastal Alaska watersheds, seasonally subsidizing river food webs in the winter when availability of autotrophic resources are limited.