Research into the contribution of multimodality to language learning is gaining momentum. While most studies pave the way for new understandings of language teaching and learning, there is an increasing demand for comprehensive assessment practices, particularly within higher education contexts. A few studies have emphasized the importance of reflecting on and establishing criteria for the assessment of multimodal literacy. This is necessary to understand students’ contributions in detail and to provide them with effective support in developing their multimodal skills. This study discusses the assessment of multimodal writing in English for Specific Purposes (ESP) contexts. It presents the design of an analytical tool for assessing multimodal texts and provided an example of its application. This tool covers assessment categories such as language use, content expression, interpersonal meaning, multimodality, and creativity and originality. As an example, we focus on the multimodal writing of a video game narrative, a genre that requires the integration of multiple modes of communication to convey meaning more effectively. Finally, this study offers pedagogical insights into the assessment of multimodal literacy in ESP.