Despite the fundamental role of unions in reducing economic inequality, social psychology still needs to incorporate the systematic study of unions concerning the contemporary theories of intergroup inequality. In this study, we do so by testing the reciprocal relationship between trust in unions and social-dominance orientation egalitarianism (SDO-E) over time. Using data from the Longitudinal Social Survey of Chile, which is a panel study with a representative sample of urban areas, we carried out a Random Intercept Cross-Lagged Panel Model with four waves (2016-2019, N = 2972-2153). We controlled the main effect by sex, age, political ideology and educational level. The results show a positive relationship between SDO-E and trust in unions within and between levels. This means that people who trust in unions also tend to have higher SDO-E (between effect), and an intraindividual increase in trust in labour unions tends to be followed by an increase in SDO-E (within effect). Trusting unions as an institution that promotes the hierarchy reduction between workers and employers helps increase people's SDO-E over time. Please refer to the Supplementary Material section to find this article's Community and Social Impact Statement.