Flexor enthesopathy is a recently recognized elbow disorder in dogs and considered to be an important differential diagnosis for elbow lameness. Primary and concomitant forms of the disease have been previously described and treatments differ for the two forms. The goal of this prospective study was to compare magnetic resonance imaging (MRI) findings for dogs with primary flexor enthesopathy (n = 17), concomitant flexor enthesopathy (n = 23), elbow dysplasia alone (n = 13), and normal elbows (n = 7). Each elbow joint underwent MRI using the same low-field scanner. Sequences included transverse and sagittal T1-weighted (before and after IV contrast), transverse and sagittal T2-weighted, and dorsal STIR. For each elbow, MRI lesions were recorded based on a consensus of two observers unaware of group status. Magnetic resonance imaging lesions involving flexor tendons were found in 100% of clinically affected joints with primary flexor enthesopathy and 96% of clinically affected joints with concomitant flexor enthesopathy. Thickened flexor muscles were the most common lesions, followed by hyperintense tendon signal and contrast enhancement. Irregular, thickened medial humeral epicondyle, edema, and calcified body lesions were less frequently observed. Magnetic resonance imaging characteristics of flexor enthesopathy were not found in normal joints or those affected by elbow dysplasia alone. No significant differences in frequencies and details of individual MRI characteristics were found between primary and concomitant flexor enthesopathy groups. Findings indicated that MRI is a sensitive technique for detection of flexor enthesopathy lesions in dogs, however, MRI characteristics do not allow differentiation of primary versus concomitant forms of the disease.