Kinship is a key factor that can influence the fitness benefits associated with social behaviors, through the operation of kin selection. A species’ patterns of dispersal, and resulting fine-scale spatial genetic structure, can mediate kin selection by altering both the capacity for kin cooperation and the intensity of kin competition. We used proximity logger collars and multilocus genotypes to investigate how genetic relatedness influences the associations of mountain brushtail possums (Trichosurus cunninghami), in the context of fine-scale spatial genetic structure. We found that spatial proximity was an important factor influencing the nocturnal encounter rate. Further, proximity was associated with relatedness between individuals, a pattern that was stronger among females than males. After proximity was accounted for, we found that possums who shared a mitochondrial haplotype associated more often and for longer during nocturnal activity. By comparison, autosomal nuclear relatedness metrics did not explain associations. This is likely to represent, in part, mother-offspring associations and suggests that kin recognition may occur through familial cues. Females also associated for longer than did males, which may be attributed to a combination of kin preference and differences between the sexes in genetic structuring. Thus, this study demonstrates the way in which social behaviors may be shaped by how kin selection and fine-scale spatial genetic structure interact.