Conceptualized in formative assessment and sociocognitive theory, peer feedback and self-regulated learning (SRL) instruction were used in this mixed-method study to investigate their effects on Thai EFL university students' essay writing ability and their self-regulation. During the 12-week online instruction period, 35 third-year students majoring in Business English at a public university were taught essay writing using the integration of peer feedback activities and SRL. An essay writing test, a self-regulation questionnaire, and semistructured interviews were used to collect data. Findings revealed that students' essay writing ability and self-regulation had significantly improved after the intervention. Our findings shed light on the feasibility and necessity incorporating the interdisciplinary dimensions of education such as instruction, assessment, and social-cognitive theory, in enhancing learners' academic performance and their lifelong learning strategies. Our findings also suggest that students can be trained in peer feedback and SRL strategies via online platforms, which has implications for the continuing tendency towards online instruction in the post-pandemic era. Pedagogical implications are also provided to help teachers improve their students' writing performance and self-regulation. [Note: The publication year (2023) shown in the citation on the PDF is incorrect. The correct year is 2024.]