Background Fixed mindsets or beliefs about the non-malleability of self-attributes are linked to a wide range of negative psychological outcomes. Its association with suicidal ideation (SI) among young people has not been explored. Objectives To examine the association of fixed mindsets of depression, anxiety, and stress and SI; and its mediating role underlying the association between depression and SI. Methods A sample of 1393 adolescents (Mage = 13.04, SD = 0.85, 640 boys) from 11 middle schools voluntarily participated in a two-wave longitudinal study before and during the COVID-19 pandemic with a 9-month interval. Results Both depressive symptoms and fixed mindsets were positively and significantly associated with concurrent and future suicidality, after controlling for demographic and socioeconomic status and previous SI. Participants with stronger fixed mindsets were more likely to have SI than those with only depressive symptoms. Also, fixed mindsets mediated the association between depressive symptoms and SI in both cross-sectional and longitudinal models. Conclusion The current study provides empirical evidence of the effects of fixed mindsets and SI and the mediating role of fixed mindset between depressive symptoms and SI among young people. Interventions to foster a growth mindset may enhance hope and reduce suicidality among adolescents.