Political solidarity is often key to addressing societal injustice. Yet social and political psychology are without a common definition or comprehensive measure of this construct, complicating advancements in this burgeoning field. To address these gaps, we advance a novel understanding and measure of this construct. We conceptualized political solidarity as a construct consisting of three factors—allyship with a minority outgroup, a connection to their cause, and a commitment to working with them to achieve social change—that can emerge within and across social groups. Five studies empirically supported our conceptualization and measure; all participants were Canadian university students. In Study 1, 1,594 participants completed the initial 30-item pool. A series of exploratory factor analyses, along with indices of factor retention, supported the three-factor model. We retained three items per factor to create the 9-item Political Solidarity Measure (PSM). This three-factor model adequately fit Study 2 data (N = 275). In Study 3 (N = 268), we found evidence of the PSM’s convergent and discriminant validity. Studies 3 and 4 assessed the PSM’s retest stability in the medium-term (three to six months; Study 3) and short-term (a three-week period; Study 4; N = 126). Finally, we demonstrate the PSM’s predictive validity in Study 5 (N = 221). Controlling for modern racism, political orientation, and gender, PSM scores predicted collective action intentions and behavior benefitting the outgroup: Participants who reported higher political solidarity donated more to the outgroup’s cause and were more likely to agree to create a message of support.