Major terrorist events, such as the recent attacks in Ankara, Sinai, and Paris, can have profound effects on a nation’s values, attitudes, and prejudices. Yet psychological evidence testing the impact of such events via data collected immediately before and after an attack is understandably rare. In the present research, we tested the independent and joint effects of threat (the July 7, 2005, London bombings) and political ideology on endorsement of moral foundations and prejudices among two nationally representative samples (combined N = 2,031) about 6 weeks before and 1 month after the London bombings. After the bombings, there was greater endorsement of the in-group foundation, lower endorsement of the fairness-reciprocity foundation, and stronger prejudices toward Muslims and immigrants. The differences in both the endorsement of the foundations and the prejudices were larger among people with a liberal orientation than among those with a conservative orientation. Furthermore, the changes in endorsement of moral foundations among liberals explained their increases in prejudice. The results highlight the value of psychological theory and research for understanding societal changes in attitudes and prejudices after major terrorist events.