Introduction: The infraspinatus and teres minor are scapulohumeral muscles that laterally rotate the arm. Multiple variations in these muscles have been reported; however, the impact of concurrent accessory infraspinatus (a-I) and teres minor (a-TM) muscles has yet to be described. The present study aims to employ gross cross-sectional and biomechanical analyses to investigate a case of coexistent a-I and a-TM muscles for structural effects, functional impacts, and clinical implications. Methods: Bilateral a-I muscles and a left a-TM muscle were discovered during routine dissection of a human cadaver. The aberrations were carefully cleaned of extraneous tissue and photographed. The left glenohumeral joint was sectioned sagittally and photographed to confirm the relation of a-I and a-TM to contiguous structures. The infraspinatus, a-I, teres minor, and a-TM muscles were examined for gross and microscopic structural features that were used to calculate maximal isometric force (Fmax) and bilateral comparability (δ). Results: The 8.66 g (bilateral mean) a-I and 13.42 g (left) a-TM inserted on the greater tubercle just distal to their namesake muscle. At the left glenohumeral joint, a-I remained distinct from infraspinatus while a-TM and teres minor exhibited connected bellies. Bilateral a-I and teres minor muscles were structurally similar (δ