With the increasing popularity of mobile technology, there is growing interest in exploring its potential for enhancing learning outcomes. This study aimed to investigate the impact of using Quizizz, a mobile application, on high school students’ grammar competence and their perceptions of the app for grammar learning. The study employed a mixed-methods approach with a quasi-experimental design, involving 92 11th-grade students at a high school in Vietnam. The experimental group received 17 weeks of grammar instruction using Quizizz, while the control group received traditional instruction. The results revealed that students in the experimental group significantly outperformed their peers in the control group, highlighting the positive impact of Quizizz on high school students’ grammar competence. Additionally, students’ perceptions of Quizizz for grammar practice were generally positive, as indicated by the questionnaire and interviews. The findings suggest that Quizizz can be an effective tool for enhancing grammar learning in high school students, and teachers should consider incorporating more mobile learning tasks into their instruction. Further research is needed to determine the generalizability of the results to a larger population of EFL learners.