Objective Osteoarthritis (OA) is the most common joint disease worldwide. The etiology of OA is varied, ranging from multifactorial to environmental to monogenic. In a condition called early‐onset OA, OA occurs at an earlier age than is typical in the general population. To our knowledge, there have been no large‐scale genetic studies of individuals with early‐onset OA. The present study was undertaken to investigate causes of monogenic OA in individuals with nonsyndromic early‐onset OA. Methods The study probands were 45 patients with nonsyndromic early‐onset OA who were referred to our skeletal disease center by skeletal dysplasia experts between 2013 and 2019. Criteria for early‐onset OA included radiographic evidence, body mass index ≤30 kg/m2, age at onset ≤50 years, and involvement of ≥1 joint site. Molecular analysis was performed with a next‐generation sequencing panel. Results We identified a genetic variant in 13 probands (29%); the affected gene was COL2A1 in 11, ACAN in 1, and SLC26A2 in 1. After familial segregation analysis, 20 additional individuals were identified. The mean ± SD age at onset of joint pain was 19.5 ± 3.9 years (95% confidence interval 3–47). Eighteen of 33 subjects (55%) with nonsyndromic early‐onset OA and a genetic variant had had at least 1 joint replacement (mean ± SD age at first joint replacement 41 ± 4.2 years; mean number of joint replacements 2.6 per individual), and 21 (45%) of the joint replacement surgeries were performed when the patient was <45 years old. Of the 20 patients age >40 years, 17 (85%) had had at least 1 joint replacement. Conclusion We confirmed that COL2A1 is the main monogenic cause of nonsyndromic early‐onset OA. However, on the basis of genetic heterogeneity of early‐onset OA, we recommend next‐generation sequencing for all individuals who undergo joint replacement prior to the age of 45 years. Lifestyle recommendations for prevention should be implemented.