Extra-pair paternity (EPP) appears to be widespread in passerine birds, but few studies have focused on suboscine species, and therefore it is unclear if this generalization can be extended to this group. In this study, we document high rates of EPP in a socially monogamous, suboscine passerine, the Scissor-tailed Flycatcher (Tyrannus forficatus) in southwest Oklahoma in the summers of 2009–2011. Averaged across years, EPP occurred in 66% of nests in our study population and accounted for 49% of all nestlings. EPP rates were variable (50–88%), but did not differ significantly among years or between two study sites. Our review of the literature yielded EPP data for only 11 other species of suboscines (0.7% of total), which we compared to EPP rates for the 83 oscine species (1.9% of total) reported in Spottiswoode and Møller (2004). EPP was as common and variable in suboscines (21.7% of young) as oscines (17.5% of young). Our study adds to our knowledge of mating systems in the understudied suboscine group, but further sampling of suboscines should be a priority, as they comprise a quarter of all passerine species and comprise a large fraction of passerines in tropical regions.