Background and objectives: Self-reported psychological variables related to pain have been posited as the major contributors to the quality of life of fibromyalgia (FM) women and should be considered when implementing therapeutic strategies among this population. The aim of this study was to explore the effect of low-pressure hyperbaric oxygen therapy (HBOT) on psychological constructs related to pain (i.e., pain catastrophism, pain acceptance, pain inflexibility, mental defeat) and quality of life in women with FM. Methods: This was a randomized controlled trial. Thirty-three women with FM were randomly allocated to a low-pressure hyperbaric oxygen therapy group (HBOTG) (n = 17), who received an 8-week intervention (5 sessions per week), and a control group (CG) (n = 16). All women were assessed at baseline (T0) and upon completion of the study (T1) for self-perceived pain intensity, pain catastrophism, pain acceptance, pain inflexibility, mental defeat and quality of life. Results: At T1, the HBOTG improved across all variables related to pain (i.e. self-perceived pain intensity, pain catastrophism, pain acceptance, pain flexibility, mental defeat) (p < 0.05) and quality of life (p < 0.05). In contrast, the CG showed no improvements in any variable. Furthermore, significant differences between the groups were found in quality of life (p < 0.05) after the intervention. Conclusions: HBOT is effective at improving the psychological constructs related to pain (i.e. pain catastrophism, pain acceptance, pain flexibility, mental defeat) and quality of life among women with FM.