Abstract Purpose To evaluate symptoms of late radiation toxicity, side effects, and quality of life in breast cancer patients treated with hyperbaric oxygen therapy (HBOT). Methods For this cohort study breast cancer patients treated with HBOT in 5 Dutch facilities were eligible for inclusion. Breast cancer patients with late radiation toxicity treated with ≥ 20 HBOT sessions from 2015 to 2019 were included. Breast and arm symptoms, pain, and quality of life were assessed by means of the EORTC QLQ-C30 and -BR23 before, immediately after, and 3 months after HBOT on a scale of 0–100. Determinants associated with persistent breast pain after HBOT were assessed. Results 1005/1280 patients were included for analysis. Pain scores decreased significantly from 43.4 before HBOT to 29.7 after 3 months (p < 0.001). Breast symptoms decreased significantly from 44.6 at baseline to 28.9 at 3 months follow-up (p < 0.001) and arm symptoms decreased significantly from 38.2 at baseline to 27.4 at 3 months follow-up (p < 0.001). All quality of life domains improved at the end of HBOT and after 3 months follow-up in comparison to baseline scores. Most prevalent side effects of HBOT were myopia (any grade, n = 576, 57.3%) and mild barotrauma (n = 179, 17.8%). Moderate/severe side effects were reported in 3.2% (n = 32) of the patients. Active smoking during HBOT and shorter time (i.e., median 17.5 vs. 22.0 months) since radiotherapy were associated with persistent breast pain after HBOT. Conclusion Breast cancer patients with late radiation toxicity reported reduced pain, breast and arm symptoms, and improved quality of life following treatment with HBOT.