The present study complements previous research findings with new data to improve our understanding of the relationship between motivational variables and academic performance in math mediated by self-regulated learning (SRL). A structural equation model with predictor (i.e., grade retention, grade level, and study time), process (i.e., perceived usefulness of SRL strategies, self-efficacy for the use of SRL strategies, and reported use of SRL), and product variables (i.e., academic achievement in mathematics) is proposed. The model was analyzed in two samples of data (calibration and validation samples). The first sample served to fit and respecify the model, and the second one was used to analyze the consistency of the findings of the first sample. A sample of 756 middle school Portuguese students participated in the current study. The results indicate that SRL is positively and significantly related to academic achievement and that the latter is, in turn, powerfully determined by perceived usefulness and self-efficacy, although students' reported use of SRL strategies decreases from 7th to 9th grades. Self-efficacy and perceived usefulness of SRL strategies were also found to decrease as grade retention increased. These results are discussed with regard to the relevance of self-efficacy and perceived usefulness of SRL strategies in increasing academic achievement.