Background: Advance care planning (ACP) is an iterative communication process about patients’ values and preferences for future care. In general practice, there are barriers to ACP at patient, general practitioner (GP), or health care system levels. A complex intervention may be necessary to reduce barriers. Aim: To evaluate the effects of a complex ACP intervention for patients with chronic, life-limiting illness in general practice (ACP-GP). Design and Setting: Cluster-randomized controlled trial in Belgian general practice. Method: ACP-GP included a patient workbook, GP training, ACP conversations, and a documentation template. The control group received usual care. Outcomes were the 15-item ACP Engagement Survey for patients and the ACP Self-Efficacy Scale for GPs. Linear mixed models evaluated differences at 3 months (T1, effectiveness evaluation) and 6 months (T2) post-baseline. Analysis was intention-to-treat. Results: 35 GPs and 95 patients were randomised. Patient ACP engagement did not differ between the intervention and control group at T1 (baseline-adjusted mean difference, 0.34; 95% CI, -0.02 to 0.69; p=0.062) or T2 (baseline-adjusted mean difference, 0.20; 95% CI, -0.17 to 0.57; p = 0.28). For GP ACP self-efficacy, there were no significant differences between groups at T1 (baseline-adjusted mean difference, 0.16; 95% CI, -0.04 to 0.35; p = 0.11) or at T2 (baseline-adjusted mean difference, 0.11; 95% CI, -0.09 to 0.31; p = 0.27). Conclusion: ACP-GP did not improve patient engagement and GP self-efficacy more than usual care. Both groups showed patterns of increase from baseline. Trial procedures and the COVID-19 may have increased awareness about ACP.