Several species of nocturnal gecko are known for their acoustic behavior in social contexts. The nocturnal house gecko, Gekko japonicus, was believed to be mute, except when threatened. We conducted behavioral encounter experiments involving same-sex and heterosexual pairs of G. japonicus. We also conducted two control experiments, chemical and blank, to confirm that acoustic behaviors are induced in the presence of conspecifics. Characteristic calls emitted by this gecko were recorded, providing the first evidence that G. japonicus uses acoustic signals for social communication. Geckos tended to call more frequently after the first physical contact with an opponent than before contact. In both control experiments, no gecko emitted calls. Whereas most characteristics of calls were similar to those reported for other gekkonid species, call intensity in this experiment was low. Males emitted calls toward both males and females, whereas females emitted calls almost exclusively toward other females. Several call variables differed significantly between intra- and intersexual calls. Our study demonstrated that G. japonicus uses acoustic signals in social contexts over short distances.