Rationale: Asthma is more common in females and more common in night shift workers. Since increasing numbers of females are becoming shift workers it is important to determine if shift work-associated asthma risk is higher in females.Objective: To determine if increasing frequency of shift work is more strongly related to prevalent asthma in females than in males.Method: We used cross-sectional data from >280 000 UK Biobank participants and logistic regression models adjusted for demographic and lifestyle factors to describe sex differences in prevalent asthma phenotypes related to shift work frequency. To obtain mechanistic insights, we explored associations with chronotype, sex hormones and menopause.Results: Compared to female day workers, female permanent night shift workers had higher covariate-adjusted odds of moderate-severe asthma (OR: 1.50 (95% CI 1.18 – 1.91)) but there was no corresponding relationship in males (OR 0.95 (95% CI 0.72 – 1.26); sex interaction p-value=0.01). Similar relationships were observed for “all asthma” and for “wheeze or whistling in the chest”. Female shift work-related asthma was driven by relationships in postmenopausal women not using Hormone Replacement Therapy (HRT) (adjusted OR: 1.89 (1.24–2.87) for moderate-severe asthma; sex interaction p-value=0.02 in permanent nightshift workers compared to dayworkers) but these relationships attenuated to the null in postmenopausal women using HRT.ConclusionOur finding that increasing shift work frequency is more strongly related to asthma in females than in males could have public health implications. Intervention studies should determine if modifying shift work schedules or HRT can reduce asthma risk in females.