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Wildfires are a worldwide disturbance with unclear implications for stream water quality. We examined stream water chemistry responses immediately (<1 month) following a wildfire by measuring over 40 constituents in four gauged coastal watersheds that burned at low to moderate severity. Three of the four watersheds also had pre‐fire concentration‐discharge data for 14 constituents: suspended sediment (SSfine), dissolved organic and inorganic carbon (DOC, DIC), specific UV absorbance (SUVA), major ions (Ca2+, K+, Mg2+, Na+, Cl−, SO42− ${{\text{SO}}_{4}}^{2-}$, NO3− ${{\text{NO}}_{3}}^{-}$, F−), and select trace elements (total dissolved Mn, Fe). In all watersheds, post‐fire stream water concentrations of SSfine, DOC, Ca2+, Cl−, and NO3− ${{\text{NO}}_{3}}^{-}$ changed when compared to pre‐fire data. Post‐fire changes in SO42− ${{\text{SO}}_{4}}^{2-}$, K+, Na+, Mg2+, DIC, SUVA, and total dissolved Fe were also found for at least two of the three streams. For constituents with detectable responses to wildfire, post‐fire changes in the slopes of concentration‐discharge relationships commonly resulted in stronger enrichment trends or weaker dilution trends, suggesting that new contributing sources were surficial or near the surface. However, a few geogenic solutes, Ca2+, Mg2+, and DIC, displayed stronger dilution trends at nearly all sites post‐fire. Moreover, fire‐induced constituent concentration changes were highly discharge and site‐dependent. These similarities and differences in across‐site stream water chemistry responses to wildfire emphasize the need for a deeper understanding of landscape‐scale changes to solute sources and pathways. Our findings also highlight the importance of being explicit about reference points for both stream discharge and pre‐fire stream water chemistry in post‐fire assessment of concentration changes. Plain Language Summary: Wildfires are becoming more common, yet the effects of wildfire on streams and their water quality remain unclear. Changes in stream water quality following a wildfire can have consequences for drinking water and aquatic ecosystems. We studied how stream water quality changed in four impacted watersheds following the 2020 CZU Lightning Complex Wildfire in central coastal California using measurements of stream water chemistry coupled with analysis of burned soils and ash from the same area. For sites with pre‐fire data, we found that wildfire impacts on stream water quality were greatest in the most extensively burned watershed. We also found that stream water quality changes were distinct across watersheds and dependent on stream discharge. Chemical measurements of potential sources of solutes, like ash and burned soils, indicate that the variability in stream water quality was consistent with contributions from ash or other burned landscape materials. The location, composition, and amount of these wildfire‐generated sources vary in space and time. This complexity can lead to some similarities as well as differences in water quality responses of different streams, like those found in our study. Key Points: Post‐fire stream water chemistry impacts were usually site, constituent, and discharge‐dependent, reflecting dynamic fire‐induced changesDespite this variability, slopes of concentration‐discharge relationships often became more positive post‐fire, implying altered surface or near‐surface sourcesHeterogeneity in fire‐generated sources and processes contributes to complexities in post‐fire stream water chemistry [ABSTRACT FROM AUTHOR]