We report the first case of conspecific calf-directed aggression in Pacific white-sided dolphins (Lagenorhynchus obliquidens) as a possible infanticide attempt in Mutsu Bay, Japan. Our observation of a 75-minute-long persistent attack on a neonate was performed by 10 attackers (4 adult males, 1 possible male, and 5 of unknown sex) and left the neonate with visible injuries. Only one individual was recorded for the entirety of the event and was regarded as the possible mother, displaying protective behaviours towards the neonate as well as being the target of coercive guarding and sexual behaviours, such as mounting, by the attackers. The observation featured a distinct group composition change in which, after 50 minutes, the attack was taken over by a new group of attackers. There was a brief overlap between the groups in which some dolphins from the first group surfaced with the second. Excluding the mother and calf, only two individuals that were observed towards the end of the first group attack remained with the second group until the end of the observation. The first group of attackers did not make further aggressive attempts on the neonate or presumed mother, and no conflict between the two groups was witnessed. The second group continued the attack with significantly increased aggression and a greater array of behavioural types than the first, often dividing into two approximately 5- to 10-m distanced subgroups-one that herded the suspected mother and another that focused on attacking the neonate. Our study analysed the frequency and variety of behavioural types used in the attack and compared them between individuals and the two separate attack groups. The group change we observed is absent from the literature on conspecific calf-directed attacks in other cetacean species and, if this is an infanticide attempt, provides new insight into the social structure of Pacific white-sided dolphins.