OBJECTIVES: Office workers represent one of the most sedentary groups. Alarmingly, more than one-third of their sitting time during workdays occurs in prolonged bouts. Sit-stand desk (SSD) interventions have been found to be effective in reducing sitting time, but heterogeneity exists amongst studies, which may be due to compensations outside the workplace. This study aimed to assess the impact of a 6-month SSD intervention on office workers' sitting and standing times during the work shift and outside work (leisure time). METHODS: A 2-arm (1:1) clustered randomized controlled trial was conducted with 38 participants randomly assigned to either the intervention or control group. The intervention included a psychoeducational session, motivational prompts, and environmental modification (SSD implemented in the workplace). The waiting-list control group maintained the usual work conditions and only received the initial session. We employed repeated measures analysis of covariance to compare changes between groups while adjusting for relevant covariates, with significance set at 5%. RESULTS: In the intervention group, there was a significant reduction in sitting time at work by 32.97 minutes, along with a notable increase in standing time by 27.88 minutes (P < .05). Although nonsignificant (P > .05), there was an increase in sitting time in the leisure context of about 23.07 minutes. CONCLUSIONS: This study underscores the effectiveness of SSD as a key strategy to mitigate sitting time among office workers. However, future interventions should consider integrating comprehensive behavioral strategies beyond the workplace to sustain potential increases in sitting time during leisure time and avoid compensatory behaviors.