Background <p>The weight-adjusted waist index (WWI) is a new way to measure central obesity, reflecting body fat distribution more effectively. The association between WWI and depression in individuals with diabetes is still not well understood. This study explores this association in diabetic patients.</p> Methods <p>This study involved adult participants diagnosed with diabetes mellitus in NHANES 2009–2018. We explored the association between WWI and depression through multivariate logistic regression analysis, subgroup analyses, interaction tests, and smoothed curve-fitting methods.</p> Results <p>In Model 3, this study found a significant positive association between WWI and depression among patients with diabetes (OR = 1.38, 95% CI: 1.17–1.63). In the smoothed curve fitting analysis, a nonlinear relationship between WWI and depression was observed, with an inflection point at 12.31. Subgroup analysis indicated that this association was influenced by the poverty income ratio (PIR), with a stronger association observed among diabetic patients with PIR ≥ 1, &lt; 3, and PIR ≥ 3.</p> Conclusions <p>Among patients with diabetes, WWI was significantly positively associated with the prevalence of depression. This association was more pronounced among diabetic patients with PIR ≥ 1, &lt; 3, and PIR ≥ 3. These findings suggest that WWI may be a relevant factor for physicians to consider when assessing depressive symptoms in patients with diabetes.</p>