Background: Autonomy-supporting interventions, such as self-determination theory and guided self-determination interventions, may improve self-management and clinical and psychosocial outcomes in people with diabetes. Such interventions have never been systematically reviewed assessing both benefits and harms and concurrently controlling the risks of random errors using trial sequential analysis methodology. This systematic review investigates the benefits and harms of self-determination theory-based interventions compared to usual care in people with diabetes. Methods: We used the Cochrane methodology. Randomized clinical trials assessing interventions theoretically based on guided self-determination or self-determination theory in any setting were eligible. A comprehensive search (latest search April 2022) was undertaken in CENTRAL, MEDLINE, Embase, LILACS, PsycINFO, SCI-EXPANDED, CINAHL, SSCI, CPCI-S, and CPCI-SSH to identify relevant trials. Two authors independently screened, extracted data, and performed risk-of-bias assessment of included trials using the Cochrane risk-of-bias tool 1.0. Our primary outcomes were quality of life, all-cause mortality, and serious adverse events. Our secondary outcomes were diabetes distress, depressive symptoms, and nonserious adverse events not considered serious. Exploratory outcomes were glycated hemoglobin and motivation (autonomy, controlled, amotivation). Outcomes were assessed at the end of the intervention (primary time point) and at maximum follow-up. The analyses were conducted using Review Manager 5.4 and Trial Sequential Analysis 0.9.5.10. Certainty of the evidence was assessed by GRADE. Results: Our search identified 5578 potentially eligible studies of which 11 randomized trials (6059 participants) were included. All trials were assessed at overall high risk of bias. We found no effect of self-determination theory-based interventions compared with usual care on quality of life (mean difference 0.00 points, 95% CI −4.85, 4.86, I ² = 0%; 225 participants, 3 trials, TSA-adjusted CI −11.83, 11.83), all-cause mortality, serious adverse events, diabetes distress, depressive symptoms, adverse events, glycated hemoglobulin A1c, or motivation (controlled). The certainty of the evidence was low to very low for all outcomes. We found beneficial effect on motivation (autonomous and amotivation; low certainty evidence). Conclusions: We found no effect of self-determination-based interventions on our primary or secondary outcomes. The evidence was of very low certainty. Systematic review registration: PROSPERO CRD42020181144