Group Psychoeducation (PE) is an effective strategy to enhance adherence to antipsychotic treatment in Bipolar Disorders (BD). However, it requires attendance to weekly sessions during a period of about 6 months. This may impede its application for those patients living far from mental health centres, resulting inequality in access to evidence-based care. Therefore, there is an increasing need to find new efficient strategies to deliver and extend PE programs to a wider population of BD patients. Mobile apps are a cost-effective way to deliver PE. In the Italian healthcare context, no evidence about the use of apps is available. The current paper presents the protocol about the development of a smartphone app to deliver PE for BD and the protocol for a trial assessing its effectiveness. In euthymic BD patients, the study will compare the adherence rates to antipsychotics between PE delivered through Bipolar mobile Application (Bip.App), group PE and a combination of both, will investigate demographic, socio-cultural and clinical predictors of lower adherence in the arms, and will investigate whether PE combined with Bip.App is associated with lower risk of recurrence of (hypo)manic and depressive episodes than group PE alone, and assess the feasibility and satisfaction for Bip.App. Participants will be recruited from mental health centres and included if they are 18-65 year-old, have primary BD in the euthymic phase, they have been prescribed a second-generation oral antipsychotic as a maintenance/prophylactic therapy for at least 1 year, they have not undergone a structured protocol of PE for BD, they have access to a smartphone and sufficient competence in using it. Participants will be excluded if they have neurological disease, mental retardation or learning disability, psychosis, limited fluency in Italian. Adherence will be assessed through count pills, blood levels, and self-reported adherence. A single-blinded parallel-group superiority multi-centre randomised controlled trial design will be used.