Background <p>This research was conducted to investigate the association between cognitive function and smartphone ownership among very old adults, a rapidly growing age group. Additionally, we conducted a longitudinal investigation as a sub-analysis to determine whether owning a smartphone affects the level of long-term care certification as a proxy outcome for cognitive decline.</p> Method <p>Data from a cross-sectional and longitudinal survey of the Kawasaki Aging and Wellbeing Project (KAWP) was used. Multivariate regression analysis was performed to examine the relationship between the Mini-Mental State Examination (MMSE) score and smartphone ownership. We also examined the association between smartphone ownership and the level of nursing care 1,278 days after the baseline.</p> Results <p>Among 483 participants aged 85–90 years at baseline, 165 (46.9%) were male, and 131 (27.1%) owned a smartphone. In adjusted regression, smartphone ownership was associated with an MMSE score of ≥ 27 (odds ratio [OR]: 1.76, 95% confidence interval [CI]: 1.13–2.77), after controlling for potential confounders, including hypertension (OR: 1.65, 95% CI: 1.01–2.69). The results revealed no relationship between smartphone ownership and long-term care level.</p> Discussion <p>Our results suggest that minimal cognitive decline in very old adults could make the ownership of a smartphone difficult.</p>