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Abstract Background High sedentary times (ST) is highly prevalent in patients with coronary artery disease (CAD), highlighting the need for behavioural change interventions that effectively reduce ST. We examined the immediate and medium-term effect of the SIT LESS intervention on changes in ST among CAD patients enrolled in cardiac rehabilitation (CR). Methods CAD patients participating in CR at 2 regional hospitals were included in this randomized controlled trial (1:1, stratified for gender and hospital). The control group received CR, whereas SIT LESS participants additionally received a 12-week hybrid behaviour change intervention. The primary outcome was the change in accelerometer-derived ST from pre-CR to post-CR and 3 months post-CR. Secondary outcomes included changes in ST and physical activity characteristics, subjective outcomes, and cardiovascular risk factors. A baseline constrained linear mixed-model was used. Results Participants (23% female; SIT LESS: n = 108, control: n = 104) were 63 ± 10 years. Greater ST reductions were found for SIT LESS compared to control post-CR (-1.7 (95% confidence interval (CI): -2.0; -1.4) versus − 1.1 (95% CI: -1.4; -0.8) h/day, pinteraction=0.009), but not at 3 months post-CR (pinteraction=0.61). Besides, larger light-intensity physical activity (LIPA) increases were found for SIT LESS compared to control post-CR (+ 1.4 (95% CI: +1.2; +1.6) versus + 1.0 (95% CI: +0.8; +1.3) h/day, pinteraction=0.020). Changes in other secondary outcomes did not differ among groups. Conclusion SIT LESS transiently reduced ST and increased LIPA, but group differences were no longer significant 3 months post-CR. These findings highlight the challenge to induce sustainable behaviour changes in CAD patients without any continued support. Trial registration Netherlands Trial Register: NL9263. Registration Date: 24 February 2021.