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. In this study, 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 distinct differences between diurnal and nocturnal associations. Diurnal (den-sharing) associations occurred within a small subset of mainly male-female dyads, whose members were socially pair-bonded. In contrast, nocturnal associations occurred between multiple individuals of both sexes. 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 but may also indicate a general preference for associating with familiar individuals. 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 social behaviors may be shaped by how kin selection and fine-scale spatial genetic structure interact.