Extra-pair paternity is of particular interest in socially monogamous species, in which there is a substantial discrepancy between social and genetic mating systems. Varied Tits Poecile varius are socially monogamous passerine birds with a distribution in Japan, Korea, and northeast China. We established paternity in all individuals of 40 broods by microsatellite genotyping. Extra-pair paternity occurred in 16 of the 40 broods (40%) and accounted for about 14.7% (37/251) of offspring, higher than the average value (11%) among socially monogamous birds. The relatedness between social partners did not correlate with the percentage of extra-pair offspring in the brood and there was no difference in heterozygosity between maternal half-siblings. As the first such investigation in this species, this study adds to our understanding of their mating system and will be useful for comparative studies of variations in extra-pair paternity rate.