The social and genetic mating systems of species can differ. This discrepancy occurs in many birds; however, only 2 studies have examined this issue for shrews, and both were conducted on species that live in temperate regions. We provide the 1st spatial and genetic data to reveal the mating system of the Asian lesser white-toothed shrew (Crocidura shantungensis) in a subtropical region. We tested the hypothesis that the mating system of shrews in the subfamily Crocidurinae is monogamous in subtropical or tropical regions with seasonal climate and resource availability. We used capture–mark–recapture methods to assess spatial organization of shrews during the reproductive season in 2006, and applied 8 microsatellite primers to perform parentage analyses and examine multiple paternity within litters. Males had larger home ranges than females, and the home ranges of resident females, but not males, overlapped with more opposite-sex than same-sex individuals. This spatial structure suggested a polygynous social mating system. Molecular analyses demonstrated that both females and males mated with multiple individuals, and the frequency of multiple paternity was 28%, which supported a promiscuous genetic mating system. Our results rejected the hypothesis that the mating system of C. shantungensis is monogamy in subtropical northern Taiwan.