<p>Teaching and learning at the university level pays special attention to developing higher order thinking skills (HOTS), where portfolio assessment can play an important role. However, research on portfolio assessment mainly focuses on writing skills development. They hardly focus on portfolio assessment for developing higher order thinking skills in writing. Adopting a qualitative case study, this research explores the effectiveness of portfolio assessment in developing HOTS among Bangladeshi undergraduate students in English as a Foreign Language (EFL) writing classes. Data were collected through portfolio folders, classroom observations, and semi-structured interviews of eight first-year undergraduate students who participated in a 3-day-long workshop on argumentative writing at a private university in Bangladesh. The data revealed that portfolio assessment enhanced students’ analysis, synthesis, and evaluation skills as they progressed through successive drafts. These skills were seen to be developed simultaneously in the same draft. Portfolio assessment created a socially rich and cognitively demanding learning environment, where students engaged in continuous feedback, reflection, self-assessment, and negotiation. Thus, by facilitating scaffolded support, encouraging active participation, and promoting self-regulated learning, portfolio assessment brought Vygotsky’s sociocultural theory into practical application in the EFL writing classroom. The study recommends wider adoption of portfolio assessment in EFL writing curricula, alongside targeted teacher training, to promote critical, reflective, and skill-oriented learning in higher education.</p>