Objectives: To investigate the long-term effects of smartphone app-based oral-muscle-strengthening training in older adults. Methods: A randomized controlled trial using a pretest-posttest control group design was conducted. Older adults were randomly assigned to an experimental group or a control group. The experimental group participated in a 12-week oral-musclestrengthening program using smartphone app content from May to August 2025. Oral health, mental health, tongue strength, and pronunciation accuracy were evaluated pre- and post-intervention. Statistical analyses were performed using a per-protocol approach. Results: Post-intervention, the experimental group showed significant improvements in the Geriatric Oral Health Assessment Index (GOHAI), O’Leary index, Löe & Silness index, and anterior tongue strength (all p<0.05). Significant betweengroup differences were observed for the GOHAI (p<0.05), Löe & Silness index (p<0.05), and anterior tongue strength (p<0.01). Sleep quality also significantly improved in the experimental group (p<0.05), although between-group differences were not significant. Pronunciation accuracy improved and phonological errors decreased, but these improvements were not statistically significant. Conclusions: Smartphone app-based oral-muscle-strengthening training may effectively improve oral health and tongue strength in older adults while enhancing accessibility and adherence through continuous participation, regardless of time and place.