Uncertainty-identity theory (Hogg, 2000, 2007, 2012) postulates that people strengthen their adherence to, and identification with, extreme ideologies when they undergo an enduring uncertainty regarding their self-definition. Concomitantly, nationalist and extreme right-wing ideologies have been associated with the attribution of a threatening character to immigrant and refugee groups. We propose that self-uncertainty precedes the perceived threat posed by the latter groups, which in turn predicts adherence to nationalist attitudes. In one correlational (Study 1; n = 169) and one experimental study (Study 2; n = 309), we tested the mediational effects of perceived realistic and symbolic threat towards immigrants on the association between self-uncertainty and nationalist attitudes (belief in national superiority, support for anti-immigration laws and intention to vote for an anti-immigration party). In both studies, perceived realistic threat emerged as the most reliable mediator between self-uncertainty and nationalist attitudes. In addition (Study 2), we found a causal effect of self-uncertainty on realistic threat. We discuss the implications of these findings for social inclusion policies based on the reduction of uncertainty generated by immigration.