At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus ('forest') lead to expansion of permafrost-free wetlands ('wetland'). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH4 ) emissions. Here, we quantify the thaw-induced increase in CH4 emissions for a boreal forest-wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long-term net carbon dioxide (CO2 ) exchange. Using nested wetland and landscape eddy covariance net CH4 flux measurements in combination with flux footprint modeling, we find that landscape CH4 emissions increase with increasing wetland-to-forest ratio. Landscape CH4 emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10°C warmer than forest soils. The cumulative growing season (May-October) wetland CH4 emission of ~13g CH4 m-2 is the dominating contribution to the landscape CH4 emission of ~7g CH4 m-2 . In contrast, forest contributions to landscape CH4 emissions appear to be negligible. The rapid wetland expansion of 0.26±0.05% yr-1 in this region causes an estimated growing season increase of 0.034±0.007g CH4 m-2 yr-1 in landscape CH4 emissions. A long-term net CO2 uptake of >200g CO2 m-2 yr-1 is required to offset the positive radiative forcing of increasing CH4 emissions until the end of the 21st century as indicated by an atmospheric CH4 and CO2 concentration model. However, long-term apparent carbon accumulation rates in similar boreal forest-wetland landscapes and eddy covariance landscape net CO2 flux measurements suggest a long-term net CO2 uptake between 49 and 157g CO2 m-2 yr-1 . Thus, thaw-induced CH4 emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.