PURPOSE Apart from a few case reports, sacroiliac joint (SIJ) involvement in osteochondromatosis has not been studied. We aimed to determine the prevalence and characteristics of such involvement using cross-sectional imaging. METHODS In this retrospective study, three observers (one junior radiologist and two musculoskeletal radiol- ogists) independently reviewed computed tomography (CT) or magnetic resonance imaging (MRI) of patients in our database who had osteochondromatosis (≥2 osteochondromas across the skele- ton) for SIJ involvement. The final decision was reached by the consensus of the two musculoskele- tal radiologists in a later joint session. RESULTS Of the 36 patients with osteochondromatosis in our database, 22 (61%) had cross-sectional im- aging covering SIJs (14 females, 8 males; age range 7–66 years; mean age 23 years; 13 MRI, 9 CT). Of these, 16 (73%) had intra-articular osteochondromas. For identifying SIJ osteochondromas on cross-sectional imaging, interobserver agreement was substantial [κ = 0.67; 95% confidence inter- val (CI): 0.34, 1.00] between the musculoskeletal radiologists and moderate (κ = 0.59; 95% CI: 0.23, 0.94) between the junior radiologist and the final consensus decision of the two musculoskeletal radiologists. In the cohort with cross-sectional imaging, the anatomical variations of the accessory SIJ (n = 6, 27%) and iliosacral complex (n = 2, 9%) were identified in six different patients with (n = 2) and without (n = 4) sacroiliac osteochondromas. CONCLUSION Cross-sectional imaging shows frequent (73%) SIJ involvement in osteochondromatosis, which, al- though a rare disorder, nevertheless needs to be considered in the differential diagnosis of such SIJ anatomical variants as the accessory SIJ and iliosacral complex. Differentiating these variants from osteochondromas is challenging in patients with osteochondromatosis.