This study is the first to explore the effects of trade unions on the gender wage gap in China. It uses the national longitudinal survey data of 2014-2020 to address endogeneity issues. The results demonstrate that the union wage premiums are higher for women than for men, while the probability of obtaining a union membership are lesser for women than for men. The decomposition results indicate that discrimination against women in non-union member groups is the primary factor contributing to the gender wage gap; further, the discrimination in obtaining union membership also widens the wage gap in China.