Labor unions play a crucial role in liberal democracies by influencing labor market and political dynamics, organizing workers’ demands and linking them to parties. However, their importance has progressively diminished in the last decades. We suggest that technological change—and industrial robotization in particular—has contributed to weakening the role of unions. We produce novel granular data on union density at the sub-national and industry level for 15 countries of western Europe over 2002-2018. Employing these data, we estimate the impact of industrial robot adoption on unionization rates. We find that regions more exposed to automation experience a decrease in union density. The decline in unionization occurs via a compositional effect, i.e., a reallocation of employment away from traditionally unionized industries towards less unionized ones. On the other hand, there is no clear evidence of a systematic reduction in union density within industries more exposed to automation.