Introduction: Dementia is caused by various diseases, including Alzheimer’s disease dementia (ADD) and dementia with Lewy bodies (DLB). We often encounter patients with dementia who have limited shoulder joint range of motion (ROM), especially those with behavioral and psychological symptoms of dementia (BPSD). But the relationship between the diseases of dementia and restricted shoulder joint ROM is currently unclear. Methods: We examined cognitive function and shoulder joint ROM in 234 new outpatients at 7 memory clinics in Japan. We assessed cognitive function using the Mini-Mental State Examination (MMSE) and Revised Hasegawa Dementia Scale (HDS-R) and BPSD using the Neuropsychiatric Inventory Questionnaire (NPI-Q). Patients were categorized by dementia diagnosis (ADD, DLB, other dementia, and control). Right, left, and total shoulder joint ROM was assessed using validated the Japanese Orthopaedic Association (JOA) score. Results: We found significant associations of lower right, left, and total shoulder joint ROM scores with male sex, advanced age, higher NPI-Q score, lower HDS-R, and MMSE scores. Little difference was found between right and left shoulder joint ROM scores. Restricted shoulder joint ROM was related to serial 7, verbal frequency domain scores on the HDS-R and repeat score on the MMSE. It was also related to the hallucinations, irritability/lability and nighttime disturbances scores on the NPI-Q. Furthermore, the dementia groups, especially the DLB group, showed worse shoulder joint ROM than the control group. Conclusions: Dementia was significantly related to restricted shoulder joint ROM. Maintaining communication and social interaction may help maintain shoulder joint ROM.