Recently, concerns have been raised about the conceptualization, measurement, factor structure, and predictive validity of second/foreign language (L2) grit. The present study aims to address these issues. Data were collected from 223 Iranian tertiary-level L2 learners at three time points over a semester. At each time point, participants completed a questionnaire assessing their L2 grit, L2 self-efficacy, conscientiousness, and self-perceived language proficiency (SPLP). Results indicated that the revised L2 grit scale consisted of two components and demonstrated measurement invariance over time. Furthermore, the L2 grit components increased over time, were longitudinally linked to SPLP, and had small-sized relationships with conscientiousness. Relative weight analysis showed that, in addition to L2 self-efficacy, the L2 grit components were significant predictors of SPLP. The findings suggest that the newly developed L2 grit scale is a psychometrically sound tool for measuring L2 grit.