Objective We estimate the causal effect of social connectedness (i.e., the frequencies of meeting with friends, relatives, or neighbors) on cognitive function (the Korean version of Mini-Mental State Exam) among Korean older adults. Methods We used longitudinal panel data collected before and during the coronavirus disease-2019 (COVID-19) to set up the fixed (FE) or random effect (RE) models. To overcome omitted variable bias or reverse causality, we used COVID-19 pandemic period as an instrumental variable to estimate the causal effect of social connectedness on cognitive function. Results Social distancing measures during the COVID-19 period decreased social interaction. The results showed that an increase in the frequency of social interaction led an increase in cognitive scores. Specifically, an increase of one unit in the frequency of meeting familiar people increased cognitive scores by 0.1470 and 0.5035 in the RE and FE models, respectively. Conclusion Social distancing policies due to the global pandemic may have increased the risk of social isolation and cognitive decline among older adults. The government and local communities need to increase their effort to develop way to connect adults through the remainder of the pandemic and beyond.