Objective: This study investigated the moderating roles of psychological flexibility and inflexibility on the relationship between climate concern and climate-related distress. It was hypothesised that higher levels of psychological flexibility would negatively moderate the relationship between climate concern and climate-related distress, while higher levels of psychological inflexibility would positively moderate the same relationship. Methods: A moderation analysis was performed in R on a sample of 771 participants from the Prolific recruitment platform. Participants completed a survey which included a climate change concern index, the Climate Change Anxiety Scale, the Multidimensional Psychological Flexibility Inventory, and the 4-item Patient Health Questionnaire. Results: The interaction term between climate-related distress and psychological inflexibility was significant (b = -0.07, SE = 0.03, p < 0.05), indicating that psychological inflexibility is a moderator of the association between concern about climate change and climate-related distress. This model explained 23% of the variance of climate related distress symptomatology. For the same level of concern about climate change, participants who reported higher psychological inflexibility, reported higher levels of climate-related distress. Discussion: Higher psychological inflexibility may be a risk factor for climate-related distress symptomatology, particularly for those who indicate greater concern about climate change. These findings contribute to the broader psychological flexibility literature in support of using ACT-based interventions for those experiencing significant distress associated with global crises such as climate change.