Objective: To explore the correlation between endometrial thickness in intrauterine insemination (IUI) cycles and clinical pregnancy outcomes. Methods: A retrospective analysis was conducted on 4 764 cycles of intrauterine sperm IUI in the Reproductive Medicine Center of Nanjing Drum Tower Hospital from 2016 to 2022. Based on the different endometrial thicknesses monitored by transvaginal ultrasound on the day of ovulation induction with human chorionic gonadotropin (hCG), the IUI cycles were divided into two groups: the endometrial thickness < 8 mm group (n=395) and the endometrial thickness ≥8 mm group (n=4 369). Differences in clinical pregnancy outcomes between the two groups were analyzed. Furthermore, a propensity score matching method was employed for a secondary analysis to control the influence of confounding variables, and univariate and multivariate Logistic regression analyses were applied to evaluate the effect of endometrial thickness on clinical pregnancy outcomes. Results: There were no statistically significant differences between the two groups in body mass index (BMI), basal follicle-stimulating hormone (FSH), duration of infertility, number of IUI cycles, IUI protocols, number of IUI, or total number of progressive motility (PR) sperm (all P > 0.05). However, there was a statistically significant difference in the type of infertility between the two groups (P < 0.05). Compared with the endometrial thickness < 8 mm group, the group with endometrial thickness ≥ 8 mm had a lower average age for both the female and male patients, and significantly higher levels of anti-Müllerian hormone (AMH) and antral follicle count (AFC) (all P < 0.05). The clinical pregnancy rate and live birth rate in the IUI cycles with endometrial thickness ≥ 8 mm were higher than those in the < 8 mm group (P < 0.05). There was no statistically significant difference in the early spontaneous abortion rate between the two groups (P > 0.05). After propensity score matching, endometrial thickness was found to have a significant effect on the live birth rate of IUI(P < 0.05). Univariate analysis indicated that factors such as female age, male age, female BMI, basal FSH, AMH, AFC, duration of infertility, endometrial thickness, and total number of PR sperm were significantly associated with the clinical outcomes of IUI(all P < 0.05). After adjusting for the significant confounding factors identified in the univariate analysis (except for basal FSH), multivariate logistic regression analysis revealed that endometrial thickness was not significantly associated with clinical pregnancy (OR=1.933, 95%CI: 0.929-4.022, P=0.078) or live birth (OR=1.838, 95%CI: 0.850-3.971, P=0.122). Conclusion: The endometrial thickness during the IUI cycle does not affect the clinical pregnancy outcome. The predictive value of endometrial thickness as a single ultrasound indicator in the IUI population is limited. Clinical decisions should comprehensively consider factors such as age, ovarian function, sperm factors, and uterine cavity factors. The decision on whether to cancel or terminate IUI should not be made solely based on endometrial thickness. However, when it comes to IUI cycles with too low endometrial thickness, clinical decisions still need to be made with caution.