The outcome of interspecific competition is affected by numerous abiotic and biotic factors. However, the effects of genetic relatedness of conspecific neighbours have not been elucidated. Here, we demonstrate that the genotype of a conspecific neighbour determines the outcome of interspecific competition. We investigated effects of genotype of neighbour sibling or non-sibling Plantago asiatica plants on competition with Trifolium repens plants. Pairs of P. asiatica, sibling or non-sibling, were grown in competition with T. repens. Sibling P. asiatica plants, but not non-sibling plants, increased leaf placement towards competitor T. repens plants, and that placement was correlated negatively with the biomass of T. repens and positively with seed production by P. asiatica. Seed production by P. asiatica plants competing with T. repens was greater in individuals paired with siblings than in those paired with non-siblings. Thus, our results show that the genotype of conspecific neighbours can alter the outcome of interspecific competition.