Front-of-package nutrition labeling (FoPNL) has been gaining prominence as a strategy to reduce the consumption of sugar-sweetened beverages as it helps consumers to clearly identify products with excess sugars. This study aimed to evaluate the efficacy of the Brazilian and Mexican FoPNL systems on simulated purchases of non-alcoholic beverages by adolescents in a fictitious online store. A total of 437 Brazilian adolescents (15-18 years old) participated in the study and were randomized into three experimental groups: control (online store with products displayed without FoPNL), Mexican FoPNL system (products with "excess in" octagonal black warning labels based on the Mexican nutritional profile), and Brazilian FoPNL system (products with "high in" magnifying glass icons following the Brazilian nutritional profile). FoPNL systems have been applied in accordance with the legislation of each country. Participants selected a non-alcoholic beverage to be purchased from 30 options in the experimental online store. After completing the simulated purchase, they responded to questionnaires regarding their perceptions of healthfulness, harmfulness, and excess nutrient content of the beverages, as well as their socioeconomic status. Compared to the control, the Mexican FoPNL system significantly reduced the prevalence of adolescents choosing beverages in the highest tertile of free sugar density (PR: 0.74, 95 %CI: 0.58-0.94), added sugar density (PR: 0.76, 95 %CI: 0.57-0.99), and energy density (PR: 0.75, 95 %CI: 0.58-0.96). Participants in the Mexican FoPNL system group selected beverages containing 11.5 %, 10.3 %, and 7.7 % less free sugars, added sugars, and energy density, respectively, compared to the control group. Regarding total quantities, the Mexican FoPNL system led to the choice of beverages with 5.3 %, 5.0 % and 3.7 % less free sugars, added sugars, and energy, respectively, than those in the control group. The Brazilian FoPNL system resulted in a 2.3 % reduction in the calories purchased compared to the control group but was ineffective in changing the amount of free and added sugars in the simulated purchases. Additionally, the Mexican FoPNL system increased the odds and the prevalence of participants perceiving sweetened beverages as harmful to health by 82 % and 30 %, respectively, compared to the control. In conclusion, the Brazilian FoPNL system was ineffective in reducing the simulated purchase quantities and densities of free sugars, added sugars, and energy. In contrast, the Mexican FoPNL system was effective, outperforming the Brazilian system by reducing the prevalence of participants choosing beverages in the highest tertiles of free sugar density, added sugar density, and energy density, and by increasing the odds and prevalence of adolescents recognizing sweetened beverages as harmful to health.