Sperm whales have a multi-level social structure based upon long-term, cooperative social units. What role kinship plays in structuring this society is poorly understood. We combined extensive association data (518 days, during 2005-2016) and genetic data (18 microsatellites and 346bp mtDNA control region sequences) for 65 individuals from 12 social units from the Eastern Caribbean to examine patterns of kinship and social behaviour. Social units were clearly matrilineally-based, evidenced by greater relatedness within social units (mean r=0.14) than between them (mean r=0.00) and uniform mtDNA haplotypes within social units. Additionally, most individuals (82.5%) had a first-degree relative in their social unit, while we found no first-degree relatives between social units. Generally and within social units, individuals associated more with their closer relatives(matrix correlations: 0.18-0.25). However, excepting a highly-related pair of social units that merged over the study period, associations between social units were not correlated with kinship (p>0.1). These results are the first to robustly demonstrate kinship’s contribution to social unit composition and association preferences, though they also reveal variability in association preferences that is unexplained by kinship. Comparisons with other matrilineal species highlight the range of possible matrilineal societies, and how they can vary between and even within species.