AbstractBackgroundRigid and inflexible behaviours are common in frontotemporal dementia (FTD), manifesting in compulsive pursuit of specific interests, routines, and rituals. Paradoxically, these changes occur alongside profound motivational disturbances including apathy and anhedonia. While posited to be related, no study to date has explored the link between motivational changes and behavioural rigidity in FTD.MethodsCarer ratings for 71 FTD participants (26 semantic dementia [SD], 45 behavioural variant [bvFTD]) were obtained on the Dimensional Apathy Scale (apathy), the Snaith‐Hamilton Pleasure Scale (hedonic tone) and the Cambridge Behavioural Inventory–Revised (CBI‐R; behavioural changes). A rigidity index was created from existing items on the CBI‐R. Whole‐brain voxel‐based morphometry was used to explore associations between rigidity and grey matter intensity in the combined FTD group.ResultsBehavioural rigidity was significantly related to apathy severity (r = 0.57) and decreased hedonic tone (r = −0.36) in the combined FTD group. Multiple linear regression revealed a significant diagnosis × hedonic tone interaction (β = −1.40), whereby lower hedonic tone predicted rigidity in SD (r = −0.65) but not in bvFTD (r = −0.18). In contrast, the relationship between rigidity and apathy did not differ between the groups (β = −0.42). At the neural level, rigidity correlated with degeneration of predominantly right‐sided frontostriatal structures including, notably, the nucleus accumbens.ConclusionsAs the first study to demonstrate a link between motivational changes and behavioural rigidity in FTD, our findings have important clinical implications. By identifying candidate mechanisms of behavioural rigidity, our findings can inform targeted interventions to manage inflexible patterns of thought and behaviour in daily life.