Fission-fusion dynamics seem to reflect individual decisions as well as temporal and spatial variations in the organization of groups of the same species. To understand the group dynamics of the Guiana dolphin, Sotalia guianensis, at Pipa Bay, Brazil, we investigated the three dimensions of a fission-fusion social system: (1) variation in spatial cohesion, (2) variation in party size, and (3) variation in party composition. Sampling took place from December 2007 to February 2009 over 176 d and we analyzed the behavioral patterns of 658 groups. Within subgroups, animals remained cohesive, particularly in groups of adults and calves. Greater cohesion was also observed during resting and fission-fusion rates were higher during milling and feeding. Groups composed of adults and juveniles showed a higher dynamics index (group size variation as a function of time) than groups composed only of adults and the fission-fusion rate was higher during dry periods. Guiana dolphin groups frequently changed their group size and composition every 20 min on average. Taking these factors into consideration, we suggest that the Guiana dolphin demonstrates fission-fusion dynamics, a pattern of behavior similar to what has been observed in other coastal odontocete species, such as Tursiops spp. and Lagenorhynchus obscurus.