Interspecific aggression is thought to be driven by competition over either shared resources or mates, with the latter facilitated by mistaken or poor species recognition. However, such aggression may potentially also be modulated by other factors, including residency in territorial species. We tested the relative strengths of intra- and interspecific aggression in the lacertid lizard Podarcis melisellensis by introducing males to both the territories of conspecific males and the territories of a sympatric lacertid, Dalmatolacerta oxycephala. We also conducted reciprocal introductions to test the effect of residency on interspecific aggression in P. melisellensis. Our results show that P. melisellensis exhibit significantly more aggression towards D. oxycephala than towards conspecifics, even though these two species do not closely resemble one another and do not exhibit extensive overlap in diet preferences. We also found an overall effect of residency on behavioural measures of aggression, as well as a clear increase in interspecific aggression towards D. oxycephala in resident relative to non-resident P. melisellensis. These results show that interspecific aggression between sympatric species can exist in the absence of breeding competition and with little resource overlap.