Structured Self-Regulated Learning (SSRL) diaries have the potential for combining the development and assessment of a student's SRL processes over time. The aim of this study was to evaluate the extent to which an SSRL diary can develop SRL and provide a reliable longitudinal assessment of SRL development in academically low-achieving undergraduate medical students. We conducted a quasi-experimental study with low-achieving medical students at Tehran University of Medical Sciences. The intervention was a weekly SSRL diary, with 21 items in two parts (before and after studying) that was integrated with weekly explicit SRL training. A repeated measures ANOVA was performed to assess the participants' SRL development across time. We conducted generalizability theory analysis in two designs; the first was to assess the reliability of the total diary score in assessing low-achieving medical students' SRL and the second was to assess the efficacy of the four-week intervention results in improving the low-achieving medical students' SRL. Each participant (n = 20) completed four SSRL diaries. There were significant positive changes during the intervention in the students' measures of total SRL, time estimation of study, time spent on study, concentration, elaboration, organization, self-monitoring, and self-evaluation. The absolute and relative generalizability coefficients for the first design, which indicates the reliability of the students' SRL scores, were 0.71 and 0.77. The absolute and relative generalizability coefficients for the second design, which presents the reliability of the improvement of students' SRL across time were 0.79 and 0.87. The results of this study suggest that SSRL diaries combined with explicit training can provide an effective and reliable method for combining the development and assessment of SRL in low-achieving medical students' SRL over time.