In animals with biparental care, maintaining a pair bond is of adaptive value because it increases reproductive success and reduces costs, such as energy and time, for finding a new mate. Hormones are important mediators of social behaviours as well as parental care, and endocrine mechanisms are therefore likely to be involved in the decision whether to stay with the same mate or separate after a breeding season. Because behavioural compatibility has been shown to increase fitness and hormones have been shown to regulate behavioural traits, here we examined whether the degree of endocrine similarity is also related to reproductive success and pair bond longevity. We used a 3-year study on free-living great tits, Parus major, to test whether mates had similar hormone levels during the parental phase. We tested specifically whether the metabolic hormone corticosterone was related to pair bond longevity and reproductive success. Baseline, but not stress-induced, corticosterone concentrations were highly correlated among members of a pair and became more similar among members of pairs that stayed together for multiple years. Pairs that increased their hormonal similarity within a season (from prebreeding to breeding) had the highest reproductive success. Pairs with more similar baseline corticosterone levels and higher reproductive success were also more likely to remain together after the breeding season. The results of this study suggest that pair bond longevity is related to endocrine similarity and reproductive success, and raise the possibility that hormonal mechanisms may be under sexual selection.