Background: In South Korea, dental sealant was included in the National Health Insurance Services (NHIS) coverage for the first molar for ages 6–14 in December 2009. The second molar was included in 2012, and the age of insurance coverage was extended to under 18 in 2013. This study aimed to verify the effectiveness of an NHIS dental sealant coverage policy for children and adolescents by comparing the changes in first molar oral health indicators before and after policy implementation.Methods: The Korea National Health and Nutrition Examination Survey data were analyzed; the fourth period (2007–2009) provided data for before and the sixth period (2013–2015) provided data for after policy implementation. The proportion of individuals with first-molar sealant, decay-missing-filled first molar permanent teeth, and single crowns in the group aged 11-20 years were calculated. Data were analyzed using chi-square for complex samples and the complex samples general linear model. In addition, complex-sample logistic regression analysis was performed to confirm the association between factors. Results: Compared with non-beneficiaries, among policy beneficiaries, sealant ownership increased by 7.7% (from 27.8 to 35.5, P<0.001), and the number of permanent teeth with sealant per capita increased by approximately 0.4 to 0.8 (P<0.001). The proportion of individuals with decay-missing-filled permanent teeth decreased by 9.1% (from 68.4 to 59.3, P<0.001), and the average decay-missing-filled permanent teeth index per person decreased by approximately 2.0 to 1.5 (P<0.001). The rate of single-crown holders decreased by 2.7% (from 8.7 to 6.0, P>0.05), and the average single-crown index decreased by approximately 0.11 to 0.08 per person(P>0.05). The number of sealants increased with age and household income (P<0.001). The mother’s education level affected sealant experience (P<0.05). The caries rate was higher in females and older respondents (P <0.001).Conclusions: The sealant covered by NHIS contributed to decreasing dental caries in Korea. However, policies that can reduce oral health inequality should also be considered, and a follow-up study is required for long-term sealant maintenance in Korea.