Peatlands are an important source of the atmospheric greenhouse gas methane (CH4). Although CH4 cycling and fluxes have been quantified for many northern peatlands, imprecision in process-based approaches to predicting CH4 emissions suggests that our understanding of underlying processes is incomplete. Microbial anaerobic oxidation of CH4 (AOM) is an important CH4 sink in marine sediments, but AOM has only recently been identified in a few nonmarine systems. We used (13)C isotope tracers and followed the fate of (13)C into CO2 and peat in order to study the geographic extent, relative importance, and biogeochemistry of AOM in 15 North American peatlands spanning a ∼1500 km latitudinal transect that varied in hydrology, vegetation, and soil chemistry. For the first time, we demonstrate that AOM is a widespread and quantitatively important process across many peatland types and that anabolic microbial assimilation of CH4-C occurs. However, AOM rate is not predicted by CH4 production rates and the primary mechanism of C assimilation remains uncertain. AOM rates are higher in fen than bog sites, suggesting electron acceptor constraints on AOM. Nevertheless, AOM rates were not correlated with porewater ion concentrations or stimulated following additions of nitrate, sulfate, or ferric iron, suggesting that an unidentified electron acceptor(s) must drive AOM in peatlands. Globally, we estimate that AOM could consume a large proportion of CH4 produced annually (1.6-49 Tg) and thereby constrain emissions and greenhouse gas forcing.