Description: Objectives Suboptimal medication adherence for infectious diseases such as tuberculosis (TB) results in poor clinical outcomes and ongoing infectivity. Directly observed therapy (DOT) is now standard of care for TB treatment monitoring but has a number of limitations. We aimed to develop and evaluate a smartphone-based system to facilitate remotely observed therapy via transmission of videos rather than in-person observation. Design We developed an integrated smartphone and web-based system (Mobile Interactive Supervised Therapy, MIST) to provide regular medication reminders and facilitate video recording of pill ingestion at predetermined timings each day, for upload and later review by a healthcare worker. We evaluated the system in a single arm, prospective study of adherence to a dietary supplement. Healthy volunteers were recruited through an online portal. Entry criteria included age ≥21 and owning an iOS or Android-based device. Participants took a dietary supplement pill once, twice or three-times a day for 2 months. We instructed them to video each pill taking episode using the system. Outcome Adherence as measured by the smartphone system and by pill count. Results 42 eligible participants were recruited (median age 24; 86% students). Videos were classified as received—confirmed pill intake (3475, 82.7% of the 4200 videos expected), received—uncertain pill intake (16, 〈1%), received—fake pill intake (31, 〈1%), not received—technical issues (223, 5.3%) or not received—assumed non-adherence (455, 10.8%). Overall median estimated participant adherence by MIST was 90.0%, similar to that obtained by pill count (93.8%). There was a good relationship between participant adherence as measured by MIST and by pill count (Spearmans r s 0.66, p〈0.001). Conclusions We have demonstrated the feasibility, acceptability and accuracy of a smartphone-based adherence support and monitoring system. The system has the potential to supplement and support the provision of DOT for TB and also to improve adherence in other conditions such as HIV and hepatitis C.