Political solidarity is often key to addressing societal inequities and injustice (Mallett, Huntsinger, Sinclair, & Swim, 2008; Scholz, 2009). Yet social psychology is without a common definition or comprehensive measure of this construct, complicating advancements in this burgeoning field. To address these gaps, across five computer-based studies of Canadian university student samples, I created and validated the Political Solidarity Measure (PSM). I conceptualized political solidarity as consisting of three factors: allyship with a disadvantaged outgroup, a connection to their cause, and a commitment to working with them to achieve social change. In Study 1, 1,594 participants completed the initial 30-item pool. A series of exploratory factor analyses, along with indices of factor retention (e.g., when m = 3, RMSEA.LB < .06, AIC Δi < 1), supported the three-factor model. I retained three items per factor to create the 9-item PSM used in subsequent studies. Confirming this factor structure, a three-factor model adequately fit data collected for Study 2 (N = 273; Robust RMSEA = 08; Robust CFI = .97); I thus retained the three-factor model. In Study 3 (N = 259), I found evidence of the PSM’s convergent validity (rs > |.19|, ps < .03), discriminant validity (rs < |.10|, ps > .23), and its medium-term (three to six month) retest reliability, r(254) = .62, p < .001. Study 4 (N = 130) also assessed retest reliability, but in the short-term (a three-week period), r(121) = .60, p < .001. Finally, I demonstrate the PSM’s predictive validity in Study 5 (N = 221). Controlling for modern racism, PSM scores predicted collective action intentions and behavior benefitting the outgroup: Participants who reported higher political solidarity donated more to the outgroup’s cause, β = .25, t(215) = 3.21, p = .002, and were more likely to agree to create a message of support, than not agree, b(SE) = 1.09 (0.27), p < .001, OR = 2.98, 95% CI [1.76, 5.05]. The PSM will enable measurement of political solidarity across issues and time, facilitate comparisons of interventions to shift political solidarity, and add to knowledge of intergroup relations and social change.