Introduction Research demonstrates that sexual dysfunction is a major concern for early-onset colorectal cancer (EOCRC) survivors and a key contributor to impaired quality of life. Despite this, sexual function outcomes after EOCRC remain poorly characterised. Aims This study aimed to assess sexual function in EOCRC survivors and to identify demographic, psychological, and treatment-related factors associated with sexual dysfunction. Methods A retrospective case–control cohort study was conducted involving EOCRC survivors diagnosed before age 50 and living in Canterbury, New Zealand, compared with age-matched controls. Sexual function was assessed using validated instruments: the Female Sexual Function Index (FSFI) and the International Index of Erectile Function (IIEF). Depression and anxiety were measured using the PHQ-9 and GAD-7 questionnaires. Logistic and linear regression analyses were used to compare rates and severity of sexual dysfunction and to explore associated factors. Results A total of 104 EOCRC survivors (56 women, 48 men) and 94 controls were included. Female EOCRC survivors had significantly higher rates of sexual dysfunction than controls (68% vs 35%, p=0.001), with significantly lower scores across five of six sexual function domains. Male survivors did not have significantly higher overall rates of erectile dysfunction compared with controls, but demonstrated significantly poorer sexual desire and orgasmic function (p<0.05). Male EOCRC survivors had significantly higher rates of depression (47.9% vs 27.4%) and anxiety (56.2% vs 25.4%) compared with controls (p<0.05). Depression, radiation, chemotherapy and advanced disease stage were significantly associated with worse sexual function. Conclusions EOCRC treatment is associated with significant impairments in sexual function, with a greater burden observed among women. In contrast, depressive symptoms were more pronounced among men, and depression, alongside cancer treatment–related factors, was a key determinant of sexual dysfunction across both sexes.