Portfolio assessment (PA) has been extensively adopted for writing development in the past three decades. Much research on PA primarily investigates students' and teachers' perceptions of its benefits, and how it influences students' motivation and general writing abilities. Despite its purported effectiveness, not much has been done to understand the relationship between PA and self-regulated learning (SRL) especially in English as a Foreign Language (EFL) or English as a Second Language (ESL) writing research. This paper contends that PA can productively foster SRL in EFL writing classrooms, and, more specifically, it develops a conceptual model of SRL within the context of writing portfolios and iterative feedback processes. Supporting evidence emphasising how PA can facilitate SRL is discussed and evaluated. The paper ends with six recommendations and implications proposing how SRL can be promulgated in EFL portfolio-based contexts. Finally, possible future research is suggested.