Purpose: This study tested whether sexual orientation disparities in depressive symptoms are partially explained by recalled childhood gender nonconformity and whether the proportion of this association explained by childhood gender nonconformity is moderated by recalled parental attitudes toward childhood gender nonconformity. Methods: A convenience sample of young adults was recruited from two Chinese online survey platforms (272 heterosexual males, 272 bisexual males, 272 gay males, 272 heterosexual females, 272 bisexual females, and 272 lesbian females). Both mediation and moderated mediation models were conducted. Results: For both sexes, bisexual and gay/lesbian individuals reported significantly higher levels of depressive symptoms than heterosexual individuals, with total effects (standardized path coefficients) ranging from 0.25 to 0.38, all ps < 0.01. These sexual orientation disparities in depressive symptoms were partially explained by childhood gender nonconformity, with indirect effects ranging from 0.08 to 0.17, all ps < 0.001. The effect of childhood gender nonconformity on depressive symptoms was significantly moderated by parental attitudes. The mediating effect of childhood gender nonconformity on sexual orientation disparities in depressive symptoms was strongest at the more negative levels (one standard deviation [SD] above the mean) of parental attitudes and weakest at more tolerant levels (one SD below the mean) of parental attitudes. Conclusions: Childhood gender nonconformity may be a partial contributor to sexual orientation disparities in depressive symptoms and this indirect effect may be moderated by parental attitudes toward childhood gender nonconformity, with the indirect effect decreasing when parental attitudes move from negative toward more tolerant levels.