Background <p>Nutritional deficiency and poor sleep quality are well-established independent risk factors for cognitive decline. However, few studies have examined their combined effects on cognitive decline. This study evaluated the synergistic association between nutritional deficiency, poor sleep quality, and cognitive decline in Chinese older adults aged 65 years and above.</p> Methods <p>Data from 10,152 participants were analyzed. Univariate and multivariate logistic regression analyses were conducted to determine the relationship between nutritional deficiency, poor sleep quality, and cognitive decline. Interactive effects were evaluated using the relative excess risk due to interaction (RERI), attributable proportion due to interaction (AP), and synergy index (S).</p> Results <p>The prevalence of cognitive decline was 13.02%. Multivariate analysis revealed that the odds ratios (ORs) for cognitive decline associated with nutritional deficiency and poor sleep quality were 3.587 (95% confidence interval [CI]: 3.125–4.118) and 1.174 (95% CI: 1.024–1.347), respectively. A significant synergistic interactive effect on cognitive decline was observed, with RERI (95% CI) = 1.037 (0.079–1.995), AP (95% CI) = 0.231 (0.052–0.411), and S (95% CI) = 1.424 (1.041–1.949). In the sex-stratified analysis, after full adjustment, the significant association between poor sleep quality and cognitive decline remained in women, but not in men (OR = 1.139, 95% CI: 0.917–1.415). Conversely, the significant additive interactive effects were only observed in men.</p> Conclusion <p>Among the elderly Chinese population, nutritional deficiency and poor sleep quality are independently associated with cognitive decline, and these two variables demonstrate a synergistic effect. These findings underscore the necessity of implementing integrated interventions that simultaneously address both nutritional and sleep-related factors in this population.</p>