Long-term records of air-pollutant emissions and atmospheric deposition, as well as water quality in streams of the Adirondack Mountains of New York, USA, indicate that chemical recovery from acidic deposition is progressing. Although Brook Trout Salvelinus fontinalis (Mitchill, 1814) have recently repopulated several lakes, the degree to which fish assemblages and individual species populations have recovered in streams of the region is unknown. The United States Geological Survey sampled acid–base chemistry and fish assemblages at 42 Adirondack region streams (2014–2021), most of which were also sampled in 2 prior periods (primarily 1979–1984 and 1999), to determine the extent and degree of chemical and biological recovery and whether responses could be linked to regional declines in acidic deposition. Between 1990 and 2021, total annual emissions of SO₂ and NO_x declined nationwide by 94% and 87%, respectively, and paralleled 88% and 70% decreases in SO₄²⁻ and NO₃⁻ concentrations, respectively, in wet deposition in the region. During this interval, mean acid neutralizing capacity increased by 19.9 µeq/L, pH increased by 0.33 units, and inorganic Al (Al_i) decreased by 6.0 µmol/L at one continuously monitored stream, whereas mean acid neutralizing capacity increased by 38.5 µeq/L and mean Al_i concentration decreased by ∼3.0 µmol/L during springtime at all 42 Adirondack region streams. Between the 1^st and 3^rd periods, mean fish-assemblage richness, density, and biomass at 40 sites increased by 112%, 236%, and 66%, respectively. Although mean Brook Trout density and biomass did not meaningfully change among periods, their distribution expanded from 20 sites in the strongly impacted sampling period (1979–1984) to 33 sites in the recovery period (2014–2021). Trends in pollutant emissions and deposition, water quality, and fish-assemblage metrics indicate aquatic ecosystems in many Adirondack mountain streams improved markedly following implementation of the 1990 Clean Air Act Amendments and recent N and S allowance-trading and emissions regulations.