With the abrupt emergence and dissemination of the COVID-19 pandemic, the traditional face-to-face classes were replaced by online classes in Iran. This quick shift has put great demands on finding and practicing new methods to teach and learn English in high schools. Owing to this urgent call, the present study follows a two-fold purpose. Firstly, it purports to disclose the effects of online portfolio assessment (PA) on developing Iranian high school students’ English writing skills. Secondly, it aims to excavate the Iranian high school students’ perceptions about the merits of online PA in cultivating their writing skills. To meet the aims, an intact second-grade class, including female students (n = 25), was chosen in Shahed public high school in Borujerd City. The class received online instruction (sixteen sessions lasting 90 minutes) based on the tenets of PA (e.g., collection, selection, and reflection). Then, a focus group interview was conducted with five of the active participants. Findings evidenced a significant improvement in the participants’ writing skills owing to the instruction. Additionally, the results of the focus group interview yielded some themes about the benefits of PA as perceived by Iranian high school students: ‘developing students’ autonomy’; ‘fostering a sense of belonging to classroom community’; ‘providing a comprehensive analysis of students’ writing proficiency’; ‘collecting empirical evidence on students’ gradual improvement in writing’; ‘training self-regulated students’; and ‘making classes student-centered by teachers-as-advisors’. Finally, a range of implications is presented to various stakeholders.
