Objective <p>Men often self-rate their mental health better than women and other gender groups. To better assess how gender is related to self-rated health, this study goes beyond the binary approach and explores how variations in self-rated mental health across multiple fixed gender categories are associated with variations along the continuous masculinity and femininity scales.</p> Methods <p>Data came from a large sample of Canadian adults (<i>n</i> = 11,712) collected from August 17 to September 19, 2021. Gender identity is measured using both fixed categories (i.e., men, women, and TNO-trans, non-binary, or other genders) as well as self-ratings of masculinity and femininity on a scale from 0 to 100. Self-rated mental health includes four categories: poor, fair, good, and excellent. Logistic regression models were used in estimations.</p> Results <p>Men reported better self-rated mental health than women and individuals who identify as trans, non-binary, or other genders. These gender gaps become non-significant once femininity and masculinity scales are taken into account. Overall, both femininity and masculinity are positively and non-linearly associated with self-rated mental health, and the positive associations get stronger at higher levels of femininity and masculinity. When separated by fixed gender categories, the accelerating positive effect of masculinity on self-rated mental health is present among both men and women, but not among TNO individuals. In contrast, the accelerating positive effect of femininity is observed only among men.</p> Conclusion <p>The gender gaps in self-rated mental health are primarily driven by the differing relationships between mental health and masculinity and femininity across gender categories. Improvement in measuring gender identity can provide valuable insights into <i>how</i> gender is a critical determinant of mental health and mental health disparities. It can also help identify those most affected and enable more targeted prevention and treatment services.</p>