While cross-sectional studies report associations between behavioral and psychological symptoms of dementia (BPSD) and nutritional status as a modifiable factor, their causal relationship remains unclear. Therefore, this study investigated the impact of nutritional status on BPSD. This study included women with mild cognitive impairment (MCI) and early-stage Alzheimer's disease (AD) from Memory Clinic, National Center for Geriatrics and Gerontology. The participants were assessed for nutritional status and BPSD using the Mini Nutritional Assessment Short-Form (MNA-SF) and the Dementia Behavior Disturbance Scale (DBD), respectively. Based on their MNA-SF scores, the subjects were classified as well-nourished, at risk of malnutrition, or malnourished. Nutritional status and change in BPSD was examined for association by univariate and multivariate linear regression analyses. This study analyzed 181 women (79 with MCI and 102 with early-stage AD). The multivariate analysis showed that the malnourished subjects or those at risk of malnutrition (54.1%) were significantly associated with increased DBD scores (β = 0.255, P = 0.003) during follow-up. In addition, multivariate regression analysis incorporating change in DBD sub-score as a dependent variable showed that the malnourished subjects or those at risk of malnutrition were associated with increased DBD sub-scores for "verbal aggressiveness/emotional disinhibition" (β = 0.247, P = 0.005). Poor nutritional status increased BPSD, especially verbal aggressiveness/emotional disinhibition, in those with MCI and early-stage AD during 2.5-year follow-up. Patients with MCI and early-stage AD may need to be assessed for nutritional status from early on, at the onset of mild cognitive decline, and require intervention to prevent worsening of BPSD. Further intervention studies in large prospective cohorts are needed to establish nutritional measures to prevent progression of BPSD in older adults with cognitive impairment.