BACKGROUND & AIMS: Bowel preparation is closely linked to the quality of colonoscopy. We investigated whether delivery of instructions via a social media app increases the quality of colonoscopy by improving adequacy of bowel preparation. METHODS: We performed a prospective study at 3 endoscopic centers in China of 770 colonoscopy outpatients (18-80 years old) with convenient access to Wechat (a widely used mobile social media app) from May through November 2014. Patients were randomly assigned to groups that received standard education along with delivery of interactive information via Wechat (n = 387) or standard education (controls, n = 383). The primary outcome was proportion of patients with adequate bowel preparation (Ottawa score <6). Secondary outcomes included rates of adenoma detection and cecal intubation, cecal intubation time, rates of incomplete compliance with instructions, and patient willingness to repeat bowel preparation. RESULTS: Demographic features were comparable between the groups. A higher proportion of patients in the group that received social media instruction had adequate bowel preparation than the control group (82.2% vs 69.5%, P < .001). Among patients with successful colonoscopies, the group that received social media instruction had lower mean total and segmental Ottawa scores (P < .05). A higher proportion of patients receiving social media instruction also had cecal intubation (97.2% vs 93.2% in controls, P = .014) and were found to have adenomas (18.6% vs 12.0% in controls, P = .012). CONCLUSIONS: Instruction via a mobile social media app, in conjunction with regular instruction, increases subjective measures of adequacy of bowel preparation. Use of the app significantly increased the proportion of patients with successful cecal intubation and in whom adenomas were detected, indicating increased quality of colonoscopy. ClinicalTrials.gov number: NCT02140827.