Self-determination is regarded as an adult outcome for people with an intellectual disability (ID). However, self-determination curricula are rarely available in Hong Kong. This paper outlines a protocol for an experimental study that examines the effectiveness of a group-based self-determination enhancement intervention for adults with mild ID and their caregivers. A randomized controlled trial with pre-test, post-test and three-month follow-up is proposed. A total of 120 participants will be randomly assigned to three conditions: self-determination enhancement group, self-determination enhancement PLUS group (with caregivers in a parallel group) and leisure activity group as a control condition. Five groups will be organized for each of the three conditions. There will be 10 sessions per group covering the core components of self-determination including self-knowledge, goal setting and attaining goals, self-regulating and adjusting plans. Components for caregivers include understanding how self-determination and REACH responding skills can support their children to exercise self-determination through positive interaction. Self-determination competencies and personal well-being will be measured at three points in time. The proposed study is the first evidence-based local study aimed at examining a culturally tailored self-determination enhancement intervention for people with ID and fills a research gap in existing interventions. If the intervention is demonstrated to be effective, it will provide new knowledge for a group-based intervention and will be used with Chinese-speaking people with ID in different parts of the world. (Trial registration: ClinicalTrials.gov: NCT05167929)