CELSR3 encodes an atypical protocadherin cell adhesion receptor that was recently identified as a high-risk gene for Tourette disorder. A putative damaging de novo variant was inserted into the mouse genome to generate an amino acid substitution within the fifth cadherin repeat. By contrast to Celsr3 constitutive null animals, mice homozygous for the R774H amino acid substitution are viable and have grossly normal forebrain development. The density of cortical and striatal interneuron subpopulations is normal, but 3D geometric analysis of cortical pyramidal neurons and striatal cholinergic interneurons revealed changes to dendritic patterning and types and distributions of spines. Furthermore, patch clamp recordings in cholinergic interneurons located within the sensorimotor striatum uncovered mild intrinsic hyperexcitability. Despite these changes, Celsr3R774H homozygous mice do not show obvious tic-like stereotypies at baseline nor motor learning impairments, but females exhibited perseverative digging behavior. Our findings show that a human mutation in CELSR3 linked to Tourette disorder is sufficient to alter dendritic patterning in the cortex and striatum and also the membrane properties of cholinergic interneurons.