Scienceârelated populismâpopulist attitudes toward scientific authorityâhas attracted growing scholarly attention. Although the SciPop Scale measures this construct, its crossânational psychometric properties remain underexamined. Using data from the Trust in Science and ScienceâRelated Populism project ( N = 69,341; 68 countries/regions) and the Swiss Science Barometer ( N = 1124), we evaluate factor structure and reliability, test multigroup invariance, apply alignment for approximate invariance, and compare scoring philosophies (Bollen vs. Goertz approach) on criterion validity. We further employ multidimensional item response theory models to provide complementary itemâlevel diagnostics, including discrimination, threshold, item information, and differential item functioning. Results show that reliability is high and a fourâfactor model fits most broadly. Full measurement invariance holds across sex, age, education, residence, political orientation, and religiosity; partial invariance appears across regions, economic development, cultural values, and regime types. Crossânational comparability is limited but improves via measurement alignment. Different scoring methods identify different weakest dimensions. For criterion validity, Bollenâbased aligned scores consistently outperform the Goertz approach, yielding higher explanatory power in crossânational and crossâgroup analyses. Finally, we provide guidelines for researchers using the SciPop Scale and future directions for measuring scienceârelated populism.