Authentic assessment has emerged as a key paradigm in 21st-century education; however, its implementation is often hindered by the limitations of conventional instruments that fail to capture the complexity of students' digital communication skills. This study aimed to develop and test the effectiveness of digital portfolios as an authentic assessment tool for 21st-century communication skills, specifically virtual collaboration, multimodal literacy, and online presentation abilities. Employing a Research and Development (R&D) approach based on the ADDIE model (Analyze, Design, Develop, Implement, Evaluate) combined with a quasi-experimental design, the study involved 84 secondary school students divided into an experimental group (digital portfolio assessment) and a control group (conventional written assessment). The digital portfolios were designed using Google Sites and Padlet platforms, incorporating the following components: (a) communication artifacts (presentation videos, infographics, podcasts), (b) self-reflection on the learning process, (c) peer and teacher feedback, and (d) evidence of online collaboration. The results demonstrated that the experimental group significantly outperformed the control group in cross-media communication skills (p < 0.001, effect size ηp² = 0.412) and the ability to reflect on their own communication strengths and weaknesses. Qualitative findings indicated that digital portfolios enhanced student agency in evaluating their own progress and provided more holistic evidence of development compared to single exam scores. The study concludes that digital portfolios serve as a valid and reliable authentic assessment tool for measuring 21st-century communication skills. Consequently, schools should integrate digital portfolios into their assessment policies and provide teacher training on designing rubrics and delivering formative feedback on student portfolios.