["Journal of Clinical Psychology, EarlyView. ", "\nABSTRACT\n\nObjectives\nIntolerance of uncertainty (IU) describes a dispositional aversion to uncertainty. Recent research has identified IU as a causal contributor to pervasive decision difficulties (indecisiveness). Given that IU and indecisiveness occur across mental disorders, understanding whether IU's causal role in indecisiveness extends to clinical populations is critical. This study therefore tested the effect of IU on indecisiveness in a clinical sample.\n\n\nMethods\nIn this pre‐registered experiment, N = 154 individuals with formally diagnosed obsessive‐compulsive and/or anxiety disorders were randomly assigned to a condition aimed at either increasing or decreasing IU. Subsequently, participants reported their current levels of (situational) IU and their indecisiveness regarding two personally relevant decisions.\n\n\nResults\nWhile the manipulation had no main effect on indecisiveness, it influenced situational IU levels, which were positively associated with indecisiveness. Mediation analysis indicated a significant indirect effect of the experimental condition on indecisiveness via situational IU.\n\n\nLimitations\nCompared to preceding research, effect sizes were small and the effect of IU on indecisiveness only manifested itself indirectly. More robust interventions may be necessary to induce more pronounced changes in IU within clinical populations. The reliance on a heterogeneous sample with various diagnoses limits the disorder‐specificity of the findings.\n\n\nConclusions\nThese results tentatively extend the causal role of IU in indecisiveness to mental disorders. The findings suggest IU as a potential approach for addressing clinically relevant indecisiveness if future research corroborates the evidence.\n"]