<p>This study aimed to examine the relationship between the disciplined mind characteristics and STEM motivation of middle school students in science class. For this purpose, quantitative research method and correlational survey design were used. Data were collected from 496 fifth, sixth, seventh and eighth grade students studying in middle schools selected by convenience sampling in Afyonkarahisar in the 2022–2023 academic year. Data collection tools included the “Demograhic Information Form”, the “Disciplined Mind Scale”, and the “STEM Motivation Scale”. The data were analyzed using descriptive statistics, Pearson correlation analysis, and hierarchical regression analysis. As a result, it was seen that the middle school students sometimes showed disciplined mind characteristics in science class and reported a moderate level of STEM motivation. It was determined that there was a positive, moderately significant relationship between the disciplined mind characteristics of middle school students in science class and their STEM motivation. When gender, grade level, maternal educational level, paternal educational level, access to a computer with an Internet connection, access to a tablet with an Internet connection, participation in any STEM project, participation in any STEM activities, and STEM career choices were all taken into account, the results of hierarchical regression analysis revealed that middle school students’ disciplined mind characteristics in science class were the most significant positive predictor of their STEM motivation. Understanding the link between students’ characteristics of a disciplined mind and their motivation for STEM subjects in science classes could inform instructional practices that encourage disciplined thinking and foster a disciplined mind. Such practices are also expected to inspire students to pursue STEM careers.</p>