This research was motivated by the low English vocabulary mastery of elementary school students, as evidenced by their difficulty recognizing, remembering, and using simple vocabulary in the learning process. This study aimed to determine students' vocabulary mastery before and after using the Lingokids application and to examine the application's effect on vocabulary mastery of second-grade students at SD Negeri 50 Bengkalis. This study used a quantitative method with a quasi-experimental design. The sample consisted of 40 students, consisting of 22 in the experimental class and 18 in the control class, selected using a purposive sampling technique. Data were collected using a pre-test and a post-test, then analyzed using descriptive statistics and tests for normality, homogeneity, and t-tests using SPSS version 27. The results showed that the average score of students in the experimental class increased from 60.23 in the pre-test to 89.77 in the post-test. The paired t-test results showed a significance value of 0.001 (Sig. < 0.05), while the independent t-test showed a significance value of < 0.001 (Sig. < 0.05). These findings indicate a significant increase in students' vocabulary mastery after using the Lingokids app, as well as better learning outcomes compared to the control class. Based on these results, it can be concluded that the Lingokids app has a positive and significant effect on elementary school students' English vocabulary mastery and can be used as an effective learning tool to improve vocabulary mastery.