OBJECTIVES: Loneliness is considered to be a crucial factor in mental health of elderly people. However, the effects of loneliness on behavioral and psychological symptoms of dementia (BPSD) have not been fully examined. The aim of this study was to investigate whether loneliness in patients with dementia is related to BPSD. METHODS: A total of 152 patients with dementia were assessed using the Neuropsychiatric Inventory (NPI-12) and the revised University of California at Los Angeles (UCLA) loneliness scale. Spearman correlation analysis and Mann-Whitney U-tests were used to examine factors associated with the revised UCLA loneliness scale. Logistic regression analysis with a forced entry method was performed to identify risk factors for BPSD. RESULTS: The revised UCLA loneliness scale score was not significantly associated with age, years of education, mini-mental state examination (MMSE) score, gender, living status, visual impairment, hearing impairment, and marital status. However, this score was a significant predictor of NPI delusion and hallucination subscale scores and Geriatric Depression Scale-15 score. The MMSE score was a significant predictor of NPI anxiety and apathy subscale scores. CONCLUSIONS: Loneliness is a risk factor for BPSD, especially for depressive symptoms and psychosis. Paying attention to loneliness in patients with dementia will help medical staff to intervene in psychiatric symptoms of these patients at an early stage.