<p>Despite the importance of students’ overall well-being in mathematics, empirical research in mathematics education has historically focused on specific outcomes related to well-being (e.g., attitudes, emotions, beliefs). A recent line of inquiry in mathematics education has begun to explore students’ overall well-being in mathematics via the construct of Mathematical Well-being (MWB). Yet, there are limited child-friendly validated instruments that classroom teachers and researchers can use to measure MWB in young students. This study examines the development and validation of the MWB scale, a brief, easy-to-use, 7-item summary measure of children’s subjective well-being in mathematics. The scale was piloted with 155 students aged 9–14 and subsequently administered to an additional 253 students aged 9–14 with demographic diversity in the USA. Six psychometric properties were evaluated, including content validity, face validity, structural validity, construct validity, internal consistency, and measurement invariance. The psychometric analysis suggests that the MWB instrument is an appropriate summary measure of subjective well-being in mathematics for students aged 9–14 in the US. The instrument offers an alternative to existing measures for researchers, teachers, and policymakers who need a brief measure for MWB.</p>