Background/Objective: Prolonged stress can overwhelm coping resources, leading people to seek mental health care. Acceptance and commitment therapy (ACT) is an intervention that enhances well-being and reduces distress, assumedly by means of increasing psychological flexibility (PF). We examined the association between a total increase in PF during an intervention and decreases in stress and increases in well-being during and after the intervention. Method: The intervention was a randomized controlled trial of an ACT-based self-help intervention. Participants were 91 individuals reporting elevated levels of work-related stress. Measurements were completed at preintervention, postintervention, and 3-month follow-up. Results: Structural equation models revealed that the total increase in PF during the intervention was negatively associated with a decrease in stress (b�=�-0.63, SE�=�0.14, p�<�.001) and positively associated with an increase in well-being during the intervention (b�=�0.48, SE�=�0.11, p�<�.001), but not with a decrease in stress (b�=�0.03, SE�=�0.27, p�>�.05) and well-being (b�=�-0.04, SE�=�0.39, p�>�.05) following the intervention. Conclusions: Our study provides empirical support for decreasing stress and promoting well-being through ACT and emphasizes the potential of PF in promoting well-being.