Quantitative ultrasound (QUS) is an inexpensive and promising tool for sensitive measurement of tendon pathology. However, few studies have reported the psychometric properties of measurements obtained using this technique for assessments of the supraspinatus tendon. The present study was undertaken to determine the variance contributed by several sources of error (participant, ultrasound operator, image analyst, analysis session) to QUS measures of the supraspinatus tendon. Transverse images of the supraspinatus tendon were captured from eleven subjects (22 shoulders) by two ultrasonographers, and each image was analyzed by two image analysts who each completed two analysis runs. Generalizability theory and intraclass correlations were used to assess the reliability of seven QUS metrics. Measures of tendon/cartilage thickness demonstrated the greatest degree of overall dependability (φ = 0.84), followed by echogenicity (φ = 0.56), variance (φ = 0.55), and entropy (φ = 0.47), suggesting that these measures of the supraspinatus tendon may be promising metrics for assessing differences in tendon health. Interrater reliability between ultrasound operators ranged from low to moderate for different QUS metrics, but using more than one image analyst and performing repeated measurement analysis runs on each image help increase reliability of QUS measures for the supraspinatus tendon.