Digital multimodal composing (DMC), a key practice in multiliteracies learning, involves students creating multimodal texts through digital tools. As literacy education broadens to include the engagement with digital multimodal texts, DMC has emerged as an innovative pedagogical approach in literacy learning. While research has established its learning benefits, comparatively less attention has been devoted to how DMC is assessed. This gap is critical because effective assessment is essential to understanding how students engage with and learn through DMC. This systematic review synthesizes current research on DMC assessment to examine the learning outcomes assessed and the approaches employed. Guided by the PRISMA protocol, 17 studies published between 2010 and 2025 were analyzed, encompassing 20 distinct DMC assessment cases. The findings reveal that DMC assessments commonly target language skills, multimodal competencies, and transversal outcomes. Self-developed or theory-informed rubrics are the predominant tools used, with over half of the assessments adopting product-oriented approaches that focus solely on final outputs. Two major challenges were then identified. First, while rubrics offer structure and practicality, their rigid and prescriptive nature limits their ability to capture the multifaceted and emergent learning outcomes of DMC. Second, applying rubrics to assess students’ DMC artefacts provides limited insight into students’ non-linear, fine-grained, and contextualized learning processes. These constraints restrict DMC assessment’s potential to meaningfully scaffold learning and provide formative feedback to students in the composing process. The paper concludes by discussing how the challenges of DMC assessment in language classrooms can be addressed through innovative approaches that harness the affordances of digital tools and spaces.