Shoulder injuries are common and frequently severe in rugby league resulting in significant time loss from playing and training. The incidence of injury may also increase in the latter stages of a season as a result of local muscular fatigue or accumulative micro-trauma, or both. Strength of rotator cuff and scapula musculature could protect against contact injuries to the shoulder. The aim of this study was to assess glenohumeral joint (GHJ) rotation strength across a season in a professional European Super League club. A prospective cohort study was used to compare GHJ rotation pre-, in-, and post-season. Twenty-nine players performed isokinetic dynamometry for concentric and eccentric muscle actions for shoulder internal and external rotation at 240°/s. A standing position with the GHJ placed in 30° of abduction and 30° of forward flexion into the scapular plane (scaption) was used to mimic actions during rugby league game play. Peak torque, traditional strength ratios, and dynamic control ratios were compared using a one-way, repeated measures analysis of variance. Ipsilateral strength ratios were indicative of selective strengthening of internal rotators (subscapularis) for concentric and eccentric muscle actions. There were no significant differences between peak torque strength parameters at any time point, regardless of whether values were considered as absolute measures or ratio standards adjusted for body mass. Similarly, strength ratios failed to reach statistical significance at any time point across the season. This null finding suggests that rugby league players are no more likely to sustain a shoulder injury in the latter stages of the season as a result of decreased GHJ strength, although these findings do not rule out sensory motor changes which could effect injury risk. Players were able to maintain rotator cuff strength across the season which is valuable to practitioners developing conditioning programmes to optimise performance and minimise the risk of injury. Increased internal rotation strength may afford protection, particularly during forced external rotation during contact in rugby league game play. From an applied perspective a decision should be made whether to maintain an imbalance, which could optimise performance and potentially reduce risk of injury by increasing tolerance of impact forces, or to try and obtain muscular re-balance which could prevent overuse injuries but negatively affect performance.