We present the first description of summer stream thermal regimes in Alaska using metrics that represent the magnitude, variability, frequency, duration, and timing of temperature events related to salmon life histories. We used cluster analysis to characterize thermal regimes present in the Matanuska-Susitna (Mat-Su) Basin based on 10 nonredundant temperature metrics and identified the most important drivers of our thermal regimes using random forests. Our results indicated four distinct thermal regimes among the 248 site-years in the Mat-Su Basin. Over 41% of site-years had cold-stable temperatures. An additional 22% of site-years had cold-variable temperatures and the latest timing of maximum stream temperatures. Twenty-eight percent of site-years had warm-variable temperatures that remained above 13°C for approximately two months. The remaining 9% of site-years had the warmest and most variable daily maximum temperatures, exceeding 18°C for almost one month, indicating potential impacts to spawning and rearing salmon. Climate and landscape drivers differentiating thermal regimes included spring and summer air temperatures, spring snowpack, summer precipitation, wetlands, and lakes. Climate change projections for 2050–2069 indicated a future shift toward warm thermal regimes and a reduced portfolio of thermal diversity. These results portend negative impacts to some salmon populations and stress the importance of prioritizing actions that maintain thermal regime diversity.