Multiple paternity may vary in frequency within populations because of changes in density. We investigated the occurrence of multiple paternity in an enclosed population of gray-sided voles (Myodes [formerly Clethrionomys] rufocanus) while focusing on the effect of density. We determined parentage from microsatellite DNA loci and mark–recapture livetrapping data. Breeding males had home ranges that covered several home ranges of reproductive females and overlapped with those of intrasexual competitors throughout the breeding season. We captured 918 weaned voles (454 females and 464 males) from 215 litters. Litters sired by multiple males (mainly 2 males) were observed throughout the breeding season, and the proportion of litters with multiple sires was 23.2% (50/215). The proportion of litters showing multiple paternity was strongly positively correlated with local density of males around estrous females. Mating success of males was related to body mass of males for mating occasions with a single sire and those with multiple sires. Taken together, our results suggest that multimale mating occurs because dominant males cannot deter subordinates from access to their mates when local density of males is high.