ABSTRACT Introduction Early Exercise Intolerance (EEI) is associated with delayed recovery and longer time to Return To Play (RTP), but this has not been established. Participants; (n = 52, male n = 30) UK university-aged rugby-union student-athletes. Methods Student-athletes completed baseline screening (July–October 2021 and 2022). The test battery was repeated within 48 h, 4, 8 and 14 days after a Sports-Related Concussion (SRC) with the Buffalo Concussion Bike or Treadmill Test to set sub-symptom heart rate threshold. Student-athletes then completed a controlled early exercise protocol in-between reassessment (days 3, 5–7 and 9–13). Those with EEI were compared to those with early-exercise tolerance. Outcome measures Post-Concussion Symptom Scale, Immediate Post-Concussion and Cognitive Test, Vestibular-Ocular Motor Screening Tool and the Revised Perceived Academic Impact Tool. Results EEI was seen throughout the initial 14-days post-SRC (23.8%, 22.4%, 25.5%. 25.0%). EEI was associated with a slower reaction time within 48 h (−0.01 (−0.030–0.043) Vs 0.06 (0.033–0.24), p = 0.004) and greater VOMS scores within 48 h; (0.00 (0.00–4.00) Vs 5.50 (2.75–9.00), p = 0.016) and 4 days (0.00 (0.00–2.00) Vs 5.00 (0.00–6.00), p = 0.044). RTP was 12.5 days longer in those with EEI at 14-days post-SRC. Conclusion EEI is prevalent following an SRC in university-aged student-athletes and was associated with delayed recovery and RTP.