Objective <p>The relationship between Glycated Hemoglobin Index (HGI) and cardiovascular disease (CVD) risk in individuals with diabetes or prediabetes remains unclear. Therefore, this study aims to investigate the relationship between baseline HGI and CVD risk in U.S. adults with diabetes or prediabetes.</p> Methods <p>This study analyzed data from 10,889 diabetic or prediabetic participants from the National Health and Nutrition Examination Survey (NHANES). Weighted multivariable regression analysis and subgroup analyses were employed to assess the relationship between HGI and CVD risk. Restricted cubic splines were used to explore nonlinear associations, along with threshold effect analysis and subgroup analyses.</p> Results <p>A total of 10,889 participants (mean age 52.82 years, 54.57% male) were included in this study. We observed a U-shaped relationship between HGI and the risk of cardiovascular disease (CVD) (P nonlinear &lt; 0.0001), heart attack (P nonlinear = 0.0006), and congestive heart failure (CHF) (P nonlinear = 0.0001). The inflection points for HGI concerning CVD, heart attack, and CHF were − 0.140, -0.447, and − 0.140, respectively. When baseline HGI exceeded these thresholds, each unit increase in HGI was significantly associated with higher risks of CVD (OR: 1.34, 95% CI: 1.23–1.48), heart attack(OR: 1.34, 95% CI: 1.20–1.51), and CHF (OR: 1.39, 95% CI: 1.22–1.58).Subgroup analysis revealed significant differences in CHF risk associated with HGI across racial groups (interaction <i>P</i> = 0.03).</p> Conclusion <p>In individuals with diabetes and prediabetes, HGI displays a U-shaped relationship with CVD, heart attack, and CHF risks, with threshold values of -0.14, -0.45, and − 0.14, respectively. HGI may serve as a more effective indicator for identifying populations at early risk for cardiovascular disease.</p>