In this digital age, digital multimodal composing (DMC) has permeated the L2 classroom as a technology-enhanced L2 writing task. Despite the learning potential of DMC for L2 students, instruments that measure L2 students’ DMC competence are rare. As such, the present study sought to develop and validate an L2 student DMC competence scale in the mainland Chinese context. Data were collected from 513 Chinese L2 students’ self-reported data on the DMC competence scale. The construct validity of the scale was scrutinized by conducting exploratory factor analysis (EFA) and confirmatory factor analysis (CFA). EFA results supported a five-factor 20-item model of the scale, which comprises digital skills, creativity, genre awareness, language use, and cohesion of modes. CFA analysis and expert judgment yielded a four-factor 15-item model, entailing digital skills, genre awareness, language use, and cohesion of modes. This study clarifies the construct of L2 student DMC competence and provides researchers with a robust instrument for measuring this construct, and teachers with a tool for evaluating the effect of DMC intervention and guiding the formative assessment of DMC projects. Students could also use the scale as a self-assessment tool to raise their metacognitive awareness of DMC competence, which helps them autonomously develop this competence.