Wetland invasion by monotypic dominant plants can alter the physicochemical and biological properties of soils that affect methane emissions, a potent greenhouse gas. We examined the effects of Typha × glauca invasion on soil methane using laboratory incubation and controlled mesocosm experiments. Typha-invaded soils collected from three Midwestern (USA) wetlands had greater methane production potential during laboratory incubation than soils dominated by native wet meadow vegetation. Ten years post-invasion of native plant-dominated mesocosms, Typha increased methane emissions at least three-fold (native: 15.0 ± 10.5 mg CH4-C m−2 h−1, median: 6.1 mg CH4-C m−2 h−1; Typha: mean: 45.9 ± 16.7 mg CH4-C m−2 h−1, median: 26.8 mg CH4-C m−2 h−1) under high (+10 cm) water levels, though methane emissions were negligible under low (–10 cm) water levels. Methane emissions were positively correlated with soil carbon, nitrogen, and aboveground biomass, all of which were greater in Typha-invaded mesocosms. Together, our data suggest that replacement of large tracts of native wetlands throughout eastern North America with monocultures of invasive Typha could alter regional methane emissions.