Background A knowledge gap exists about the risk of cancer in individuals with intellectual disability (ID). The primary aim of this study was to estimate the cancer risk among individuals with ID compared to individuals without ID. Methods and findings We conducted a population-based cohort study of all children live-born in Sweden between 1974 and 2013 and whose mothers were born in a Nordic country. All individuals were followed from birth until cancer diagnosis, emigration, death, or 31 December 2016 (up to age 43 years), whichever came first. Incident cancers were identified from the Swedish Cancer Register. We fitted Cox regression models to calculate hazard ratios (HRs) and 95% confidence intervals (CIs) as measures of cancer risk in relation to ID after adjusting for several potential confounders. We analyzed ID by severity, as well as idiopathic ID and syndromic ID separately. We performed a sibling comparison to investigate familial confounding. The study cohort included a total of 3,531,305 individuals, including 27,956 (0.8%) individuals diagnosed with ID. Compared with the reference group (individuals without ID and without a full sibling with ID), individuals with ID were in general more likely to be male. The median follow-up time was 8.9 and 23.0 years for individuals with ID and individuals without ID, respectively. A total of 188 cancer cases were identified among individuals with ID (incidence rate [IR], 62 per 1,000 person-years), and 24,960 among individuals in the reference group (IR, 31 per 1,000 person-years). A statistically significantly increased risk was observed for any cancer (HR 1.57, 95% CI 1.35–1.82; P < 0.001), as well as for several cancer types, including cancers of the esophagus (HR 28.4, 95% CI 6.2–130.6; P < 0.001), stomach (HR 6.1, 95% CI 1.5–24.9; P = 0.013), small intestine (HR 12.0, 95% CI 2.9–50.1; P < 0.001), colon (HR 2.0, 95% CI 1.0–4.1; P = 0.045), pancreas (HR 6.0, 95% CI 1.5–24.8; P = 0.013), uterus (HR 11.7, 95% CI 1.5–90.7; P = 0.019), kidney (HR 4.4, 95% CI 2.0–9.8; P < 0.001), central nervous system (HR 2.7, 95% CI 2.0–3.7; P < 0.001), and other or unspecified sites (HR 4.8, 95% CI 1.8–12.9; P = 0.002), as well as acute lymphoid leukemia (HR 2.4, 95% CI 1.3–4.4; P = 0.003) and acute myeloid leukemia (HR 3.0, 95% CI 1.4–6.4; P = 0.004). Cancer risk was not modified by ID severity or sex but was higher for syndromic ID. The sibling comparison showed little support for familial confounding. The main study limitations were the limited statistical power for the analyses of specific cancer types, and the potential for underestimation of the studied associations (e.g., due to potential underdetection or delayed diagnosis of cancer among individuals with ID). Conclusions In this study, we found that individuals with ID showed an increased risk of any cancer, as well as of several specific cancer types. These findings suggest that extended surveillance and early intervention for cancer among individuals with ID are warranted.