Background Dietary management plays a crucial role in the treatment of patients with ulcerative colitis (UC). While various e-services provide dietary advice, the long-term dietary management requires continuous monitoring and dynamic adjustment to accommodate the evolving nature of the disease and meet the patients’ nutritional needs. Consequently, the development of a novel dietary management tool that incorporates diet tracking, personalized nutritional feedback, and evidence-based advice becomes imperative. This study aims to address this need by developing a WeChat applet called “HealthyGut” specifically designed for the dietary management of UC patients, and evaluate its feasibility, acceptability, and preliminary efficacy. Methods A total of 134 UC patients were equally allocated into the intervention group (receiving a 12-week mobile-based dietary management via HealthyGut) and control group (receiving a paper-based food diary and routine advice). The feasibility outcomes were recruitment, retention, engagement, satisfaction, and acceptability in the intervention group. Dietary intakes were effective outcomes. Results Both groups had satisfactory retention rates (89.6% and 77.6%, respectively). The System Usability Scale in the intervention group yielded “good usability” with a mean score of 79.63 (SD 7.39), and all participants reported good user experiences and perceived benefits after using HealthyGut. At week 12, intervention responders reported significantly higher daily energy intake than control group (Z = −3.089, p = 0.002). Conclusions and Implications The results display that HealthyGut as a dietary management tool is feasible and accepted by UC patients, and it may help them make healthier food choices. Larger sample studies should be considered in the future.