Nursing students are exposed to numerous academic and clinical stressors that require effective psychological adaptation. Emotion regulation and cognitive flexibility are two important psychological constructs associated with coping and resilience; however, the relationship between these variables has received limited attention among Iranian nursing students. This study aimed to investigate the relationship between emotion regulation strategies and cognitive flexibility among undergraduate nursing students. In this descriptive correlational cross-sectional study, 82 second- to fourth-year undergraduate nursing students at Torbat Heydariyeh University of Medical Sciences, Iran, participated during the 2025–2026 academic year. Data were collected using the Emotion Regulation Questionnaire (ERQ) and the Cognitive Flexibility Inventory (CFI). Statistical analyses were performed using IBM SPSS Statistics for Windows, Version 26. Spearman’s rank-order correlation coefficient and independent-samples t-test were used for data analysis, and the level of statistical significance was set at P < 0.05. The mean total score of cognitive flexibility was 100.54 ± 13.58, and the mean total score of emotion regulation was 45.34 ± 8.25. A significant positive correlation was found between total emotion regulation and total cognitive flexibility (rho = 0.31, P = 0.004). Cognitive reappraisal showed significant positive correlations with total cognitive flexibility (rho = 0.48, P < 0.001) and the Alternatives subscale (rho = 0.57, P < 0.001), whereas expressive suppression was not significantly associated with any dimension of cognitive flexibility. No significant gender differences were observed in the main study variables. The findings suggest that cognitive reappraisal is positively associated with cognitive flexibility among nursing students. Strengthening adaptive emotion regulation strategies may contribute to improved cognitive flexibility and better adaptation to educational and clinical challenges.