Autism spectrum disorder is associated with sensory processing alterations, such as sensory hyper- and hypo-responsiveness. Twin studies are scarce in this field, but they are necessary in order to disentangle the genetic and environmental contributions to this association. Furthermore, it is unclear how different neurodevelopmental/psychiatric conditions contribute to altering sensory processing. We investigated the association between autistic traits/autism spectrum disorder diagnosis and sensory processing alterations in twins ( N = 269), using the adult/adolescent sensory profile, which differentiates four sub-domains: Low Registration, Sensation Seeking, Sensory Sensitivity, and Sensation Avoiding. While the associations between autistic traits and Low Registration and Sensation Avoiding persisted within monozygotic (genetically identical) twins, Sensory Sensitivity was only associated with autistic traits within dizygotic twins. In multivariate analyses with different neurodevelopmental/psychiatric diagnoses as predictor variables, autism spectrum disorder and attention deficit hyperactivity disorder were the strongest predictors for two adult/adolescent sensory profile sub-domains each. The results suggest that the association between autistic traits and Sensory Sensitivity is influenced by genetics while non-shared environmental factors influence the associations between autistic traits and Low Registration and Sensation Avoiding. They further indicate that altered sensory processing is not specific to autism spectrum disorder, while autism spectrum disorder is a strong predictor of certain sensory processing alterations, even when controlling for other (comorbid) neurodevelopmental/psychiatric conditions. Lay abstract Individuals diagnosed with autism often describe that they process sensory information differently from others, and many experience sensory issues as problematic. For instance, an increased sensitivity to smells or sounds can make participating in social settings challenging. While sensory issues are now part of the diagnostic criteria for autism, they also co-occur with other psychiatric diagnoses such as attention deficit hyperactivity disorder and anxiety disorders. It is unclear to what extent the relationship between autism and alterations in sensory processing are due to genetics or environment. In addition, more research is needed on how autism, as compared to other diagnoses, is associated with sensory issues. Using a twin study, we found that genetic factors influenced self-reported reactivity to sensory stimuli in autism while environmental factors influenced other sensory issues (e.g. difficulties in detecting or differentiating sensory input). Hence, sensory hyper-reactivity might be an early onset core feature of autism, while other domains of alterations in sensory processing might develop later, influenced by the environment. Moreover, autism was more strongly associated with sensory issues related to increased sensitivity/reactivity as compared to other psychiatric diagnoses. However, attention deficit hyperactivity disorder was more strongly related to deficits in detecting/differentiating sensory stimuli and with an increased drive to seek sensory input. Our results indicate that sensory issues are not specific to autism, but that some aspects of altered sensory processing are more relevant for autism than for other diagnoses.