Political extremism represents a collective threat to open and democratic societies, a hazard that has grown in recent times. The main goal of the present research is to test one novel construct, the Monopoly on Truth (MoT), by examining its association with illegal and violent extremist acts in a cross-cultural setting. Thus, the association between MoT and extremism has been tested in several countries with different cultures and socio-political landscapes: Brazil, Colombia and Spain (N = 785). Results support configural and metric invariance of the MoT scale, supporting construct validity. The obtained data also replicate the original validation by showing a curvilinear relation between ideology and MoT. Crucially, the correlational analysis shows that MoT is associated with extremism in all countries participating in the study, even when controlling for ideology. This research contributes to the literature on the psychological underpinnings of extremism by introducing a novel correlate of illegal and violent political action based on a notion of moral superiority unbiased by ideology.