Writing comprehensibly, dynamically, and persuasively in a foreign language is a significant challenge for learners. Written communication assesses language and writing progress for various summative and developmental goals. Scenario-based assessment (SBA), as one of the methodologies of classroom-based assessment (CBA), aims to elicit both learners' autonomous and interconnected linguistic abilities in a hypothetical real-world language use by putting the language in the setting and context for the assessment. The SBA model's effectiveness in improving argumentative essays in the Iranian EFL context is underexplored. Previous studies have focused on learners' first-language literary skills but not on argumentative writing, neglecting its influence on specific aspects of writing performance. This study examined the effect of SBA on the Iranian EFL learners' writing performance and its components, namely task response, coherence and cohesion, lexical resource, and grammatical range and accuracy. The researchers selected 40 EFL learners based on convenience sampling and randomly divided them into experimental and control groups. The experimental group received treatment through SBA, while the control group was instructed through the traditional teacher-centered method. Both groups received pre-tests and post-tests on writing performance. The collected data were analyzed through MANCOVA analysis, and the findings showed that the SBA group's participants outperformed the control group in writing performance. It also revealed that the SBA model significantly improved coherence and cohesion and lexical resources. SBA can acquire further backing and function as a proficient instructional and evaluative framework among teachers, educators, stakeholders, curriculum planners, and syllabus designers for EFL learners.