Although numerous studies have investigated the effect of green space on mental health, as yet, little is known about the association between green space and suicide. To address this deficit, we conducted the first Japan-wide study of the relationship between green space and suicide mortality. Results from spatial analyses of municipality-level panel data for the 1975–2014 period that included 886,440 suicide deaths with a fixed-effect estimator showed that green space was associated with suicide mortality, and that this relationship was conditioned by the form of greenness, level of urbanity and demographic characteristics. In densely-populated cities, park density (parks per 1000 of the population) was associated with reduced suicide mortality among females aged 18 and above and among males aged 18 to 39 and aged 65 and above. In small- and medium-sized cities, park coverage (% by area) was linked to fewer suicide deaths among middle-aged and older females (aged 40 and above). In contrast, in non-cities (rural areas), parks were not associated with suicide mortality whereas woodland coverage (% by area) was linked to reduced suicide deaths among middle-aged and older males (aged 40 and above). Our findings suggest that urban green space and rural forest coverage may have a protective effect against self-harming behavior. Future suicide prevention efforts should consider an increased greening of the residential environment in terms of both availability and accessibility, especially with better designs that accommodate population needs and local conditions.