DescriptionGrit, defined as perseverance and passion for long-term goals (Duckworth, Peterson, Matthews, & Kelly, 2007), is a character strength that may be a promising target for improving academic achievement. Despite grit’s popularity and its increasing application to underserved students, there is no published literature examining the psychometric properties of the Short Grit Scale (Grit-S) or validating the construct in underserved student populations. The current study sought to determine whether previous research on the psychometric properties and construct validity of grit replicated in a low-income, majority Latino middle school. To better understand the grit construct, the study also investigated how grit interacts with experience of an academic setback to predict academic achievement. In addition, the study sought to clarify the role social-emotional learning (SEL) skills play in explaining the impact of grit on academic achievement. Students from a large urban middle school completed the self-report Grit-S and teachers completed observational measures of observed student SEL skills. Student demographics and grades data were obtained from the school’s online database. Results indicated that although a two-factor structure (“Interest” and “Effort”) replicated in this sample, the two factors behaved in a manner inconsistent with previous research. The reliability coefficients of the full Grit-S and of each subscale were also very low. Hierarchical regressions showed that the “Interest” factor was a consistent predictor of academic achievement, whereas the “Effort” factor was not. Experiencing an academic setback was found to be a consistent predictor of grades; however, the interaction of grit and setbacks was only significant for models predicting grades in Math and students’ Best subject. Notably, the significant interactions were inconsistent with hypothesized relationships in that higher grit in students who had experienced a setback predicted lower grades. Finally, mediation analyses supported a partial mediation of grit’s impact on academic achievement by SEL skills, which is the first empirical support for how grit operates to impact academic achievement. Results from this study show that more research is needed to clarify the construct validity of grit in disadvantaged, Latino middle school students.