Indo-Pacific bottlenose dolphins (Tursiops aduncus) off Amakusa-Shimoshima, approximately 200 individuals, form relatively large groups frequently exceeding 100 individuals and show high site fidelity to the area around Tsuji Island, northern coast of Amakusa-Shimoshima. This suggests that individual dolphins may have long interaction times with many other individuals. Consequently, competition between males is likely to be high and formation of alliances may be expected. However, this has not yet been confirmed. With photo-identification data collected between 2010 and 2014, we examined individual associations. Pairs of males formed significantly non-random associations for multiple years, and were seen surrounding females, many of whom were considered to be receptive at that time. Our results suggest that male Indo-Pacific bottlenose dolphins form alliances in this population, where dolphins form large groups and show high site-fidelity.