Objectives To determine the prevalence of burnout in doctors practising obstetrics and gynaecology, and assess the association with defensive medical practice and self-reported well-being. Design Nationwide online cross-sectional survey study; December 2017–March 2018. Setting Hospitals in the UK. Participants 5661 practising obstetrics and gynaecology consultants, specialty and associate specialist doctors and trainees registered with the Royal College of Obstetricians and Gynaecologists. Primary and secondary outcome measures Prevalence of burnout using the Maslach Burnout Inventory and defensive medical practice (avoiding cases or procedures, overprescribing, over-referral) using a 12-item questionnaire. The odds ratios (OR) of burnout with defensive medical practice and self-reported well-being. Results 3102/5661 doctors (55%) completed the survey. 3073/3102 (99%) met the inclusion criteria (1462 consultants, 1357 trainees and 254 specialty and associate specialist doctors). 1116/3073 (36%) doctors met the burnout criteria, with levels highest amongst trainees (580/1357 (43%)). 258/1116 (23%) doctors with burnout reported increased defensive practice compared with 142/1957 (7%) without (adjusted OR 4.35, 95% CI 3.46 to 5.49). ORs of burnout with well-being items varied between 1.38 and 6.37, and were highest for anxiety (3.59, 95% CI 3.07 to 4.21), depression (4.05, 95% CI 3.26 to 5.04) and suicidal thoughts (6.37, 95% CI 95% CI 3.95 to 10.7). In multivariable logistic regression, being of younger age, white or ‘other’ ethnicity, and graduating with a medical degree from the UK or Ireland had the strongest associations with burnout. Conclusions High levels of burnout were observed in obstetricians and gynaecologists and particularly among trainees. Burnout was associated with both increased defensive medical practice and worse doctor well-being. These findings have implications for the well-being and retention of doctors as well as the quality of patient care, and may help to inform the content of future interventions aimed at preventing burnout and improving patient safety.