OBJECTIVES: Lip-seal strength, which represents the muscle strength of the lips, appears to chiefly contribute to mastication and pronunciation. However, the functional characteristics of lip-seal strength in adults are still undefined. The present study aimed to understand not only the distribution of lip-seal strength in adult men and women but also the effect of age on this strength and identify oral motor functions correlated with lip-seal strength. MATERIALS AND METHODS: The subjects included 339 participants (men: 170, age 39.2 ± 18.2 years; women: 169, age 43.1 ± 19.7 years). Oral motor function was evaluated for lip-seal strength, oral diadochokinesis (ODK), tongue pressure, occlusal force, and masticatory performance. Statistical analyses included the Shapiro-Wilk, Mann-Whitney U, and Jonckheere-Terpstra tests, in addition to the Spearman's correlation analysis and curvilinear regression analysis. RESULTS: Lip-seal strength did not have a normal distribution (p < 0.001). The mean ± standard deviation and median (first quartile, third quartile) of lip-seal strength were 11.2 ± 3.4 and 10.9 (8.7, 13.2)N for the whole sample, 12.3 ± 3.4 and 11.9 (9.4, 14.4)N for men, and 10.2 ± 3.0 and 9.9 (8.0, 12.0)N for women. A significant difference was observed in lip-seal strength between men and women (p < 0.001). Oral motor functions showed a marked correlation with lip-seal strength, including tongue pressure, occlusal force, and masticatory performance and ODK (/pa/ and /ta/), tongue pressure, and masticatory ability in men and women, respectively. In women, lip-seal strength declined with increase in age. CONCLUSIONS: Lip-seal strength was non-normally distributed in both men and women, and lip-seal strength was affected by age only in women. Lip-seal strength and multiple oral motor functions were significantly correlated. Because the indicators of perioral muscle strength and performance were correlated with lip-seal strength, lip-seal strength may also partially reflect the condition of the perioral muscles.