IMPORTANCE: Monoclonal gammopathy of undetermined significance (MGUS) is associated with an increased risk of cardiovascular disease. Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) have demonstrated cardiorenal benefits in patients with type 2 diabetes, but their effectiveness in patients with MGUS remains unexplored. OBJECTIVE: To assess the effectiveness of GLP-1 RAs for primary prevention of major adverse cardiovascular and cerebrovascular events (MACCE) in patients with MGUS and diabetes. DESIGN, SETTING, AND PARTICIPANTS: This retrospective cohort study used a propensity score-matched analysis of data from the TriNetX Global Database, encompassing patients diagnosed with diabetes and MGUS between January 1, 2018, and January 13, 2023. Patients with prior heart failure (HF), ischemic heart disease, coronary revascularization, or stroke or transient ischemic attack before MGUS diagnosis were excluded. The cohort was divided into 2 groups: GLP-1 RA users and nonusers at baseline. After 1:1 propensity score matching, GLP-1 RA users and nonusers were compared up to 5 years from the MGUS diagnosis date. Data analyses were completed January 19, 2025. EXPOSURE: GLP-1 RA use within 1 year before MGUS diagnosis. MAIN OUTCOMES AND MEASURES: The primary end point was MACCE, defined as a composite of all-cause mortality, new-onset HF, acute coronary syndrome, and stroke or transient ischemic attack. Secondary end points included individual MACCE components, decompensated HF, and acute kidney injury or end-stage kidney disease. RESULTS: A total of 4871 patients with MGUS (mean [SD] age, 68.9 [10.1] years; 2366 [48.5%] male) were included (473 GLP-1 RA users and 4398 non-users). A total of 460 users were matched to 460 nonusers, with balanced characteristics (mean [SD] age, 65.0 [10.6] vs 65.1 [11.0] years; 229 [49.7%] male vs 234 [50.8%] male), including 14 patients (3.0%) vs 13 patients (2.8%) identifying as Asian, 8 (21.3%) vs 92 (20.0%) as Black or African American, 25 patients (5.4%) vs 20 patients (4.3%) as Hispanic or Latino, and 243 patients (52.8%) vs 250 patients (54.3%) as White. GLP-1 RA use was associated with a significantly lower risk of MACCE (hazard ratio [HR], 0.75; 95% CI, 0.60-0.93). Significant reductions were also observed in all-cause mortality (HR, 0.57; 95% CI, 0.37-0.87), new-onset HF (HR, 0.69; 95% CI, 0.54-0.90), decompensated HF (HR, 0.60; 95% CI, 0.43-0.84), and acute kidney injury or end-stage kidney disease (HR, 0.73; 95% CI, 0.57-0.92). CONCLUSIONS AND RELEVANCE: The findings of this cohort study of GLP-1 RA use vs no use in patients with MGUS and diabetes suggest the potential of GLP-1 RA for primary prevention of MACCE. These findings warrant further investigation in prospective randomized trials.