This study explores the development of gender ideologies across adolescence and the transition to adulthood in Germany and investigates the relevance of first romantic relationship experiences in shaping gender beliefs. Integrating the life course perspective with the theoretical framework of gender as a social structure and psychological theories, we extend the literature by following adolescents from age 15 to about age 20 across the transition to adulthood and by differentiating between young women and men from different immigrant and non-immigrant backgrounds, who may be affected differently by gender-related expectations. Using the representative and ethnically diverse German sample of the Children of Immigrants Longitudinal Survey in Four European Countries, we conducted fixed-effects regression analyses (n = 1474). First, our findings show that young people become increasingly egalitarian during adolescence, irrespective of gender and immigrant origin. Second, for young women, romantic relationship experiences of moderate and longer durations are significantly associated with slower increases in egalitarianism. For most young men, romantic relationship experiences do not affect their gender ideologies. One exception is the group of Turkish-origin men, who change their gender ideologies less towards egalitarianism with increasing relationship durations than other groups of young men. The findings suggest that, especially for young women, early romantic relationships may be crucial in shaping their gender beliefs, which subsequently predict important educational, occupational, and family choices. • Young people become increasingly gender-egalitarian during adolescence. • Heterosexual relationship experience can shape adolescents' gender ideologies. • Relationship experience slows down the trend towards egalitarianism for young women. • Among young men, only those of Turkish-origin experience a traditionalising effect. [ABSTRACT FROM AUTHOR]