We develop and validate a short self-report measure of test anxiety, the Trait Test Anxiety Inventory – Short (TTAI-S) following the Kane (2013) validation framework. Data were collected from three independent samples of young adults in the US ( N = 629; Mage = 22.25 years). Evidence was gathered to support three aspects of the validity argument (i.e., scoring, extrapolation, and generalization). Good internal consistency and confirmed structure of a single factor supported scoring inferences. Scalar measurement invariance between different samples (Internet vs. undergraduate students) and demographic subgroups (i.e., gender, race/ethnicity, and parental educational attainment) provided evidence for generalization inferences. Significant associations between the TTAI-S score and theoretically relevant (state test anxiety, performance expectation, and self-confidence in math) and weaker associations with less relevant constructs (enjoyment, motivation, and values in learning math) substantiated extrapolation inferences. Having established measurement invariance, we examined demographic differences and found that students historically underserved or underrepresented in STEM disciplines reported greater test anxiety than their counterparts. These findings support the validity of the TTAI-S, a concise measure that is easy to administer and easy to score. The TTAI-S may be used to further investigate trait test anxiety for a diverse population, particularly factors that may contribute to or mitigate group differences.