Background: The development of digital technology increased adolescents’ access to online gambling, which potentially had negative impacts on mental health. Low levels of adolescents’ knowledge about online gambling could increase the risk of involvement and mental health problems. This study aimed to determine the relationship between adolescents’ knowledge of online gambling and their mental health at STT Dharma Laksana Bakti Banjar Temesi, Gianyar. Methods: This study employed a quantitative design with an analytic correlational approach and a cross-sectional method. The population consisted of 179 adolescents, with 124 respondents selected using purposive sampling. Data were analyzed using univariate and bivariate analyses with the Spearman rho statistical test. Results: The majority of respondents were male (101 respondents; 81.5%) and were aged 18–21 years (63 respondents; 50.8%). Most adolescents had a moderate level of knowledge about online gambling (59 respondents; 47.6%). The majority of respondents had low mental health status (101 respondents; 81.5%). The Spearman rho test showed a significant positive correlation between the level of knowledge about online gambling and adolescents’ mental health, with a p-value of 0.038 (p < 0.05) and an r-table value of 0.186, indicating that adolescents with better knowledge tended to have better mental health. Conclusion: There was a significant relationship between adolescents’ knowledge of online gambling and mental health. Increasing adolescents’ knowledge about the risks and impacts of online gambling was important as a preventive effort to maintain and improve adolescents’ mental health, with the involvement of families, health professionals, and educational institutions.