Same-sex marriage legalisation improves mental health among sexual and gender minority (SGM) people. In Japan, where same-sex marriage is not legalised, some municipalities have introduced ‘partnership certificate’ schemes (PCSs) to offer same-sex couples social recognition without legal recognition. We examined whether mental health status in SGM adults differed between municipalities with and without PCSs.

We used cross-sectional data on adults aged ≥18 years from a nationwide internet survey conducted in September and October 2022. We estimated the relative difference by comparing the differences in mental distress and suicidal thoughts reported by SGM adults residing in municipalities with and without PCSs to the corresponding differences among cisgender heterosexual adults.

Mental distress (Kessler 6 Scale score ≥5) and self-reported suicidal thoughts, adjusted for sociodemographic characteristics, health status and municipal characteristics.

We analysed 28 106 participants (mean (SD) age, 48.2 (17.3) years), including 3169 (11.3%) SGM individuals. Despite no significant difference in mental distress, we found a statistically significant difference in rates of self-reported suicidal thoughts between municipalities with and without the scheme for the SGM group (average marginal effects (AME), +5.1 percentage points (pp); 95% CI +0.4 to +9.8 pp) and the cisgender heterosexual group (AME, –1.6 pp; 95% CI –3.0 to –0.1 pp; relative difference, +6.7 pp; 95% CI +1.8 to +11.5 pp).

Residing in a municipality with PCSs was not associated with better mental health among SGM people in Japan. This may reflect limited scheme uptake or perceptions of inequality with legal marriage. These schemes without national legislation may not adequately address internalised stigma or promote acceptance, and they may have lowered the self-esteem of SGM people, leading to worse mental health. Further research is needed to clarify the underlying mechanisms of this association.