To investigate the characteristics of peripheral lymphocyte subsets in patients with insomnia disorder (ID) and their associations with cognitive function. Methods: A total of 39 drug-naïve patients with ID and 51 healthy controls (HCs) were recruited from Wuhan First Hospital between April 2022 and March 2023. Cognitive function was assessed using the Montreal Cognitive Assessment (MoCA) and the Stroop Color-Word Test. Flow cytometry was performed to determine the proportions and absolute counts of peripheral lymphocyte subsets in both groups. Between-group comparisons, correlation analyses, and multivariable linear regression analyses were conducted to evaluate the relationships between peripheral lymphocyte subsets and cognitive function. Results: Compared with HCs, the ID group had significantly lower MoCA total scores and poorer performance in multiple cognitive domains (P<0.05). In the Stroop test, the ID group showed reduced accuracy and prolonged Stroop-D reaction time (P<0.05). In addition, the ID group exhibited significantly increased proportions and absolute counts of T cells, helper T lymphocytes (Th cells), B cells, and total T+B+NK cells (P<0.05), whereas the proportion of NK cells was significantly decreased (P= 0.001). Correlation analyses further showed that the MoCA total score and several domain scores were negatively correlated with the proportions and absolute counts of Th cells and B cells (P<0.05), but positively correlated with the proportion of NK cells (P<0.05). Moreover, the accuracy and reaction time in Stroop-C and Stroop-D were significantly correlated with T-cell- and Th-cell-related parameters (P<0.05). Multivariable linear regression analysis showed that, after adjustment for age, education level, and BMI, Th-cell proportion, Th-cell absolute count, the Th/cytotoxic T lymphocyte (CTL) ratio, B-cell absolute count, NK-cell proportion, and total T+B+NK absolute count were independently associated with the MoCA total score. Conclusion: Patients with ID showed mild multidomain cognitive impairment, predominantly involving executive function, accompanied by a peripheral immune profile characterized by adaptive immune predominance. This imbalance in peripheral lymphocyte subsets was significantly associated with cognitive decline and may represent a potential biomarker of insomnia-related cognitive impairment.