Evidence regarding cancer risk after borderline ovarian tumors (BOTs) is limited. We conducted a nationwide cohort study examining the incidence of nonovarian cancers in women with serous or mucinous BOTs compared with the general female population with up to 41 years of follow-up. Through the nationwide Pathology Registry, we identified nearly 5000 women with BOTs (2506 serous and 2493 mucinous) in Denmark, 1978 to 2018. We computed standardized incidence ratios (SIRs) with 95% confidence intervals (CIs) as relative risk estimates of specific nonovarian cancers. Compared with general female population rates, women with serous BOTs had increased rates of particularly malignant melanoma (SIR = 1.9; 95% CI: 1.3-2.6), thyroid cancer (SIR = 3.0; 95% CI: 1.4-5.4) and myeloid leukemia (SIR = 3.2; 95% CI: 1.5-5.8), and women with mucinous BOTs had elevated rates of lung cancer (SIR = 1.7; 95% CI: 1.3-2.1), pancreatic cancer (SIR = 1.9; 95% CI: 1.2-2.9) and myeloid leukemia (SIR = 2.3; 95% CI: 0.9-4.7). We found no convincing association with neither breast nor colorectal cancer in women with BOTs. This is the first large nationwide study showing that women with specific types of BOTs have increased risks of several nonovarian cancers, likely due to some shared risk factors or genetic characteristics.