This study described, assessed and correlated ultrasonographic, computed tomographic, and histological findings in the sacroiliac joints of adult Warmblood horse cadavers. In total, 25 joints from 15 horses were examined post-mortem using transrectal ultrasonography and helical computed tomography. Findings on computed tomography were graded in the caudal joint third (caudally) and for the entire joint as mild, moderate, or severe. In total, 11 joints from nine horses were evaluated histologically. All joints (100%) showed abnormalities on computed tomography, and 92% (23/25) displayed abnormal ultrasonographic findings. The most common ultrasonographic findings were osteophytes (92%), joint effusion (76%), and sacral/iliac bone modeling (76%). Computed tomography revealed osteophytes (92% caudally, 100% overall), sclerosis (72% caudally, 88% overall), subchondral bone lesions (60% caudally, 88% overall), and enthesophytes (60% caudally, 68% overall). The most severe CT findings occurred caudally (44%), whereas 24% occurred cranially,16% in the mid portion of the SIJ, and 16% were multifocally present in the SIJ Histological analysis showed degenerative changes in the cartilage, subchondral bone, and the joint capsule. Horses with more pronounced imaging abnormalities also showed corresponding histological degeneration. Significant correlations were found between computed tomographic findings caudally and in the entire joint (r = 0.915, < 0.001, = 25), and between imaging and histological findings (computed tomography: r = 0.731, = 0.011, = 11; ultrasonography: r = 0.67, = 0.024, = 11). Non-significant correlations were observed between mean ultrasonographic and computed-tomographic grades (r = 0.35, = 0.087, = 25). Findings suggest these structural changes may reflect adaptation to joint loading. Transrectal ultrasonography appears suitable as a first-line diagnostic tool. However, future studies are essential to compare the diagnostic imaging findings of SIJs in asymptomatic and symptomatic horses to elucidate their clinical relevance.