Purpose: Restricted and repetitive behaviours are a core feature of autism diagnoses but have not been widely studied in adulthood. This study examined the rates of and associations between repetitive behaviours and sensory sensitivity in autistic and non-autistic adults; and whether repetitive behaviours described as "stimming" impacted coping with difficulties (self-efficacy). Methods: Diagnosed autistic (n = 182), undiagnosed autistic (n = 163) and non-autistic (n = 146) adults completed online measures of repetitive behaviours, sensory sensitivity, and self-efficacy for when able and not able to stim. Results: Repetitive behaviours and sensory sensitivity correlated significantly in each group, although ratings were higher in autistic compared to non-autistic groups. When people were able to stim, no differences between the groups were observed on self-efficacy ratings. However when unable to stim, autistic people reported lower self-efficacy than non-autistic people. Conclusions: Results suggest that repetitive behaviours are significantly associate with sensory sensitivities. Rather than repetitive behaviours being viewed as negative, stimming was associated with increased self-efficacy. Results suggest that stimming may have beneficial effects. Further work is needed to better understand how repetitive behaviours and stimming manifest in adulthood, how they change over time and their effects for autistic adults.