BACKGROUND: People with acquired brain injury (ABI) experience communication breakdown in everyday interactions many years after injury, negatively impacting social and vocational relationships. Communication partner training (CPT) is a recommended intervention approach in communication rehabilitation after ABI. Access to long-term services is essential, both in rural and remote locations. Digital health has potential to overcome the challenges of travel and improve cost efficiencies, processes, and clinical outcomes. OBJECTIVE: We aimed to collaboratively develop a novel, multimodal web-based CPT intervention (convers-ABI-lity) with key stakeholders and evaluate its feasibility for improving conversation skills after brain injury. METHODS: This mixed methods study consisted of 3 key stages guided by the Integrate, Design, Assess, and Share (IDEAS) framework for developing effective digital health interventions. Stage 1 included the integration of current end-user needs and perspectives with key treatment and theoretical components of existing evidence-based interventions, TBI Express and TBIconneCT. Stage 2 included the iterative design of convers-ABI-lity with feedback from end-user interviews (n=22) analyzed using content analysis. Participants were individuals with ABI, family members, health professionals, and paid support workers. Stage 3 included the evaluation of the feasibility through a proof-of-concept study (n=3). A total of 3 dyads (a person with ABI and their communication partner [CP]) completed 7 weeks of convers-ABI-lity, guided by a clinician. The outcome measures included blinded ratings of conversation samples and self-report measures. We analyzed postintervention participant interviews using content analysis to inform further intervention refinement and development. RESULTS: Collaborative and iterative design and development during stages 1 and 2 resulted in the development of convers-ABI-lity. Results in stage 3 indicated positive changes in the blinded ratings of conversation samples for the participants with traumatic brain injury and their CPs. Statistically reliable positive changes were also observed in the self-report measures of social communication skills and quality of life. Intervention participants endorsed aspects of convers-ABI-lity, such as its complementary nature, self-guided web-based modules, clinician sessions, engaging content, and novel features. They reported the intervention to be relevant to their personal experience with cognitive-communication disorders. CONCLUSIONS: This study presents the outcome of using the IDEAS framework to guide the development of a web-based multimodal CPT intervention with input from key stakeholders. The results indicate promising outcomes for improving the conversation skills of people with ABI and their CPs. Further evaluation of intervention effectiveness and efficacy using a larger sample size is required.