Clinical evidence has suggested that oral function is associated with cognitive, physical, and nutritional status of older people. A smaller volume of masseter, a crucial muscle for mastication, was associated with frailty. It has remained unknown if smaller masseter is associated with cognitive impairment. The current study investigated the association between masseter muscle volume, nutritional status, and cognitive status in older people. We recruited 19 patients with mild cognitive impairment (MCI), 15 patients with Alzheimer's disease (AD), and 28 sex and age-matched non-cognitive impairment (non-CI) older subjects. The number of missing teeth (NMT), masticatory performance (MP), maximal hand-grip force (MGF), and calf circumference (CC) were assessed. The masseter volume index (MVI) was calculated based on the masseter volume measured using magnetic resonance imaging. The MVI was significantly lower in the AD group, compared to the MCI as well as the non-CI group. Multiple regression analyses revealed that the MVI was significantly associated with nutritional status (as indexed by CC) in the analysis of combination of NMT, MP, and the MVI. Moreover, the MVI was a significant predictor of CC only in patients with cognitive impairment (i.e., MCI+AD) but not in the non-CI group. Our findings suggested that in addition to the NMT and MP, masseter volume is a critical oral factor associated with cognitive impairment. Reduction of MVI should be carefully monitored for patients with dementia and frailty, to whom a lower MVI may indicate worse nutrient intake.