Background: There is concern that assisted reproductive technology (ART) may increase ovarian cancer risk, but previous studies are inconclusive. We compared ovarian cancer risk for women who gave birth after ART vs natural conception. Methods: Through linkage of nationwide registry data, we followed 3,303,880 initially nulliparous women in Denmark (1994–2014), Finland (1990–2014), Norway (1984–2015) and Sweden (1985–2015) from first pregnancy ≥22 weeks to ovarian cancer, emigration, death or end of follow-up (2014/2015). We estimated hazard ratios (HRs), adjusting for age, parity, maternal birth year and country, and for body mass index and smoking in subsamples. Results: Mean age at first birth was 27.7 years. During a mean follow-up of 14.4 person-years, 2683 participants (0.08%) developed ovarian cancer; 135 after ART and 2548 after natural conception only (incidence rates 11.6 and 5.5 per 100,000 person-years, respectively). The risk was higher for women who ever gave birth after ART (HR 1.70, 95% confidence interval 1.42–2.03) compared to natural conception. Associations were stronger for conventional in vitro fertilisation than for intracytoplasmic sperm injection. Conclusions: Among parous women, ART-conception was associated with a higher risk of ovarian cancer than natural conception. Further studies should decipher whether this is causal or confounded by infertility or other factors.