When a loved one is missing, those left behind need to be able to tolerate high levels of uncertainty, sometimes for sustained periods of time. This study aimed to examine the relationship between intolerance of uncertainty (IU) and psychological symptoms among people with a missing loved one, and tested the mediating role of emotion regulation difficulties and psychological inflexibility. A cross-sectional sample of 110 people with a missing loved one completed a questionnaire containing demographics and measures of IU, difficulties in emotion regulation, psychological inflexibility and psychological symptoms. Results indicated that psychological inflexibility was a significant mediator of the associations between IU and psychological distress, prolonged grief and posttraumatic stress symptoms. Emotion regulation difficulties did not emerge as a significant mediator in any of the models. Findings suggest that stronger tendencies to respond negatively to uncertain situations are associated with increased attempts to avoid uncomfortable internal experiences (e.g., thoughts, feelings, memories or sensations) which, in turn, exacerbates psychological symptom levels. The findings have implications for developing interventions aimed at strengthening the ability to tolerate uncertainty and addressing psychological inflexibility among people with a missing loved one.