Introduction. Mild cognitive impairment (MCI) is a preclinical, transitional stage of cognitive impairment. With timely treatment of MCI, it is possible to prevent the progression of cognitive dysfunction and delay the onset of dementia. A violation of the concentration of sodium ions can contribute to impaired perfusion of the brain, therefore, sodium homeostasis and the human body's ability to maintain normal concentrations of sodium ions in blood plasma are important. Aim: to study the relationship of natriemia and frailty with MCI in elderly and senile people. Materials and methods: the study included 403 people over the age of 60. Complaints, medical history, measurement of anthropometric parameters, and serum sodium levels were collected from all study participants. The presence of frailty syndrome was assessed using the «Age-not-Hindrance» questionnaire, and cognitive impairment was assessed using the Montreal Cognitive Function Assessment Scale (MoCA test). Results. Participants with the lowest tercile of serum sodium had a higher risk of developing MCI than participants from groups with other terciles with an average tercile (Odds ratio (OR) = 1.91; 95% confidence interval (CI): 1.14–3.20). With serum sodium levels below 143 mmol/L, the risk of MCI was 1.38 (95% CI: 1.03–1.84) per 1 standard deviation (SD) reduction. Frailty is significantly associated with an increased risk of MCI (OR = 5.28, 95% CI: 2.70–10.30). Moreover, frail participants with either the lowest (OR = 5.53, 95% CI: 2.08–14.67) or the highest tercile (OR = 3.48, 95% CI: 1.20–10.05) serum sodium had a significant risk of MCI. Conclusion. Both low and high serum sodium levels significantly increase the risk of developing MCI in frail geriatric patients. In this regard, timely personal development of a set of therapeutic and diagnostic measures to correct serum sodium levels and frailty syndrome in elderly and senile people is necessary.