Objective Sexuality is an important aspect of human identity and contributes significantly to the quality of life in women as well as in men. Impairment in sexual health after vaginal delivery is a major concern for many women. We aimed to examine the association between degree of perineal tear and sexual function 12 months postpartum. Design A prospective cohort study Setting Four Danish hospitals between July 2015 and January 2019 Participants A total of 554 primiparous women: 191 with no/labia/first-degree tears, 189 with second-degree tears and 174 with third-degree/fourth-degree tears. Baseline data were obtained 2 weeks postpartum by a questionnaire and a clinical examination. Sexual function was evaluated 12 months postpartum by an electronic questionnaire (Pelvic Organ Prolapse/Urinary Incontinence Sexual Function Questionnaire (PISQ-12)) and a clinical examination. Primary outcome measures Total PISQ-12 score and dyspareunia Results Episiotomy was performed in 54 cases and 95 women had an operative vaginal delivery. The proportion of women with dyspareunia was 25%, 38% and 53% of women with no/labia/first-degree, second-degree or third-degree/fourth-degree tears, respectively. Compared with women with no/labia/first-degree tears, women with second-degree or third-degree/fourth-degree tears had a higher risk of dyspareunia (adjusted relative risk (aRR) 2.05; 95% CI 1.51 to 2.78 and aRR 2.09; 95% CI 1.55 to 2.81, respectively). Women with third-degree/fourth-degree tears had a higher mean PISQ-12 score (12.2) than women with no/labia/first-degree tears (10.4). Conclusions Impairment of sexual health is common among primiparous women after vaginal delivery. At 12 months postpartum, more than half of the women with a third-degree/fourth-degree tear experienced dyspareunia. Women delivering with no/labia/first-degree tears reported the best outcomes overall. Thus, it is important to minimise the extent of perineal trauma and to counsel about sexuality during and after pregnancy.