The present study explores the sources of validity evidence of the mathematics self-concept construct in Programme for International Student Assessment (PISA) 2022 with a focus on concurrent, discriminant, and predictive evidence. Mathematics self-concept was provided with a single index variable in PISA 2022. Using only the U.S. students who participated in PISA 2022, we included 4552 students. While we grounded our analyses in the "Standards" (2014), we aimed to evaluate whether the mathematics self-concept is related to mathematics motivation, distinct from mathematics self-efficacy measures and predicts mathematics achievement. Results revealed a positive association between mathematics self-concept and motivation (X[superscript 2] = 86008.92, p < 0.001, [phi] = 0.153), suggesting concurrent evidence. Significant but negligible correlations with two separate mathematics self-efficacy measures (r[subscript pb] = -0.078 and -0.073, p < 0.001) supported discriminant evidence. Multilevel modeling analysis demonstrated that mathematics self-concept significantly predicted mathematics performance ([beta] = 20.20, p < 0.001), supporting predictive validity. Validity evidence for the mathematics self-concept scale in PISA 2022 could serve as a basis for research focused on enhancing student outcomes. Implications, limitations, and future directions are discussed.