Relatives are expected to cooperate more and compete less. In symbiotic partnerships, hosts may benefit from interacting with highly-related symbionts because there is less conflict. This has been difficult to test empirically. We used the arbuscular mycorrhizal symbiosis to study the effects of fungal relatedness on host and fungal benefits, creating fungal networks varying in relatedness (self vs. non-self) between two host plants, in both soil and in-vitro systems. To determine how fungal relatedness affected overall transfer of nutrients to host roots, we fluorescently tagged phosphorus and quantified resource distribution. We imaged fungal networks, and related fractal dimension to fungal growth strategies. We found that decreased relatedness was associated with increased fungal growth and complexity, lower transport of nutrients across the fungal network, and lower plant benefit, likely an outcome of increased fungal competition. More generally, we demonstrate how relatedness among symbionts can mediate the overall benefits of symbiotic partnerships.