Objective: Increasing rates of Caesarean sections has led to concerns on long-term effects on the offspring's health and it has been hypothesized that Caesarean section induced differences in the child's microbiota could potentially increase the risk of mental disorders. Methods: Nationwide Danish cohort study of 2,196,687 births between 1980 and 2015, with 38.5 million observation-years. Exposure was 'Caesarean Section' and outcome was the child's risk of any mental disorder. Absolute and relative risks were estimated using inverse probability weighting to adjust for age, calendar time, and confounding variables while accounting for the competing risk of death. Results: Caesarean section (n=364,908, 16.6%), compared to vaginal birth, was associated with a small relative risk increase of 8% (RR, 1.08; 95% CI, 1.04-1.13; n=44,352) for development of any in-patient psychiatric admission at age 36 for the offspring, and with a small absolute risk difference of 0.47% (95% CI, 0.23-0.76). When looking at all in-patient, out-patient and ER psychiatric contacts among people born after 1995 the effect was diminished (RR, 1.04; 95% CI, 0.99-1.09; n=15,211). The risk was comparable when comparing prelabour versus intrapartum Caesarean section (RR, 0.98; 95% CI, 0.90-1.08) and acute versus planned Caesarean section (RR, 1.00; 95% CI, 0.80-1.29). Conclusion: Birth by Caesarean section was associated with only a very slightly increased risk of any in-patient psychiatric admission for the offspring and diminished even further when including all psychiatric contacts. The very small associations observed may be explained by unmeasured confounding and is unlikely to be of substantial clinical relevance.