The abnormal expression and function of myelin‐related proteins contribute to nervous system dysfunction associated with neuropsychiatric disorders; however, the underlying mechanism of this remains unclear. We found here that breast carcinoma amplified sequence 1 (BCAS1), a basic protein abundant in the brain, was expressed specifically in oligodendrocytes and Schwann cells, and that its expression level was decreased by demyelination. This suggests that BCAS1 is a novel myelin‐associated protein. BCAS1 knockout mice displayed schizophrenia‐like behavioral abnormalities and a tendency toward reduced anxiety‐like behaviors. Moreover, we found that the loss of BCAS1 specifically induced hypomyelination and the expression of inflammation‐related genes in the brain. These observations provide a novel insight into the functional link between oligodendrocytes and inflammation and/or abnormal behaviors.