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Background Regular faecal immunochemical test (FIT) screening can potentially reduce colorectal cancer (CRC) incidence and mortality. However, after the first FIT, most people do not continue regular annual or biannual testing. A theory-driven WhatsApp chatbot (TWC) uses specific psychological or behavioural theory to increase engagement, which may help to improve adherence.We aim to assess the effectiveness of TWC in increasing the FIT adherence rate compared to the standard of care (SoC).Methods We conducted a prospective, open-label, two-arm, parallel, randomized controlled trial. Asymptomatic subjects with prior negative FIT results under the government-subsidized population-based CRC screening program (the Program) who were due for repeat FIT with access to WhatsApp were randomized 1:1 to SoC and TWC groups. The SoC group received only a standard text reminder delivered by the Program, while the TWC group received SC and intervention from a health belief model and protection motivation TWC. Study outcomes included 1) repeat FIT adherence rate defined as the return of completed FIT kits by 3-month post-intervention; 2) mean days to screening completion; and 3) reasons for adherence and nonadherence.Results From February to August 2025, a total of 370 participants were recruited and randomized. There was no difference between their baseline characteristics ( IDDF2026-ABS-0067 Table 1). The FIT adherence rate was significantly higher in TWC group, when compared to SoC group (49.2% vs 27.0%, p<0.001). The mean days [standard deviation, SD] to screening completion were similar in both groups (46.5 [32.9] vs 59.3 [49.0], p=0.100). The most common reasons for adherence and nonadherence were perceived susceptibility and a busy schedule, respectively.Conclusions The novel, theory-driven WhatsApp chatbot improved longitudinal adherence of FIT screening through enhancing health beliefs about CRC.Abstract IDDF2026-ABS-0067 Table 1Participant characteristics and study outcomesWC group N=185SC group N=185p-valueMean age (SD)62.0 (5.0)61.8 (5.2)0.815Male gender (%)89 (48.1)96 (51.6)0.467Mean BMI (SD)23.0 (2.9)22.7 (2.1)0.243Ever smoking (%)25 (13.5)24 (13)0.878Alcohol consumption (%)19 (10.4)20 (10.9)0.865Family history of CRC (%)16 (8.6)16 (8.6)1.000Hypertension (%)36 (19.7)39 (21.3)0.698Diabetes (%)12 (6.6)8 (4.4)0.358Repeat FIT adherence rate (%)91 (49.2)50 (27.0)<0.01Mean days to screening completion (SD)46.5 (32.9)59.3 (49.0)0.100BMI: body mass index; CRC: colorectal cancer; FIT: fecal immunochemical test; SC: standard care; TWC: Theory-based WhatsApp chatbot