The effect of radical resection of male rectal cancer on sexual function has been the focus of attention. Despite this, there remains a dearth of robust evidence regarding the influence of robotic radical resection of rectal cancer on postoperative sexual function, particularly in men diagnosed at an early age. This study aims to explore the implications of robotic radical resection of rectal cancer on sexual function in early-onset overweight male patients diagnosed with this disease. A retrospective analysis was performed on male patients under 50 years old and over 20 years old who were diagnosed with rectal cancer (cT1-3N0M0) and underwent surgical treatment in the First Affiliated Hospital of Nanchang University from May 2015 to August 2020. Sexual function was evaluated by the International Index of Erectile Function (IIEF) test and scored at 1, 3, 6, and 12 months postoperatively. The sexual function of traditional laparoscopic radical resection of rectal cancer (L-RE) and robotic radical resection of rectal cancer (R-RE) were compared. According to body mass index, L-RE and R-RE groups were further divided into normal body weight groups (LN-RE and RN-RE) and overweight groups (LO-RE and RO-RE), and the sexual function of each group was compared successively. Neither L-RE nor R-RE patients had significant differences in number of lymph nodes removed, tumour size, pathological TNM stage, or first exhaust time or time to eat liquids. The OS and DFS of the L-RE and R-RE groups, as well as the LO-RE and RO-RE groups, did not differ statistically after the logarithmic rank test (P > 0.05). IIEF scores in both the L-RE and R-RE groups declined sharply 1 month after surgery and then steadily increased. The R-RE group's IIEF scores significantly recovered in 6 months, compared to 12 months in the L-RE group. In comparison of subgroups, the results of sexual function in the LN-RE and RN-RE groups were similar to those in the L-RE and R-RE groups. Conversely, the RO-RE group showed slightly improved sexual function recovery than the LO-RE group 3 and 6 months post-surgery. 12 months after surgery, no significant difference was observed between the two groups. With similar long-term oncology outcomes, the robot-assisted surgical approach provided better protection of sexual function for men with early-onset rectal cancer, especially for those with a higher body mass index (BMI).