The academic contexts of Teaching English as a Foreign Language in Ecuador demand that teachers search for strategies to foster learners’ language skills development. Writing is the most complex skill to develop because it needs formal context and assertive guidance. Therefore, this study focused on utilizing the Portfolio assessment strategy to enhance writing skills by constructing a qualitative case study, in which a young learner from a regular course at a language institute participated in an onsite modality for nine weeks. A writing activity, considered the pre-test, took place in the first week, followed by additional writing activities developed every week. Self-evaluations were also developed, and a final writing activity constituted the post-test. The corresponding analysis showed that the learner’s writing skill improved. Thus, linguistic errors of the semantic, morphological, and discourse levels obtained the highest correction. The assessment criteria showed a positive result, highlighting that the organization and communicative achievement criteria obtained better results compared to the content and language criteria, but these also achieved significant improvements. Consequently, this study concludes that the Portfolio assessment strategy contributes to the learners’ writing skill improvement in EFL classes.