Students who exhibit mature number sense make sense of numbers and operations, use reasoning to notice patterns, and flexibly choose effective problem-solving strategies (McIntosh et al., 1997, https://ro.ecu.edu.au/ecuworks/6819). Due to its dispositional nature, mature number sense is typically measured through in-depth interviews or tests of strategy usage. Yet, the lack of an efficient, rigorously developed measure has made it difficult to collect systematic, replicable evidence on students' mature number sense. To address this, we developed a brief assessment of mature number sense. The present study provides additional convergent evidence of validity for this measure with US students in grades 3-8 (8-14 years old). We compared middle school (N = 40) and upper elementary school (N = 41) scores from the brief assessment with an established, time-intensive measure (Yang, 2019, https://doi-org.bibliotheek.ehb.be/10.1007/ s10649-018-9874-8) and an in-depth interview of student strategy usage (Markovits & Sowder, 1994, https://doi-org.bibliotheek.ehb.be/10.2307/749290). We found strong correlations (r > 0.7) across all three measures, and this held even when controlling for students' arithmetic scores (pr > 0.6). Researchers and educators can now use the brief assessment to investigate students' mathematical thinking and advance knowledge of a key aspect of mathematical cognition.