The main aim of this study was to explore the effect of the serious mobile game Jumo the Jumper on the vocabulary acquisition of primary school English as a Foreign Language (EFL) students and their attitudes toward an EFL course. The main research design of the study was a quasi-experimental design with nonequivalent control groups that included pre-test and post-test control groups. Twenty students in the experimental group played the serious mobile game aimed at teaching English vocabulary about the "animals" unit for four weeks, while 20 students in the control group received traditional, curriculum-based tuition. Based on the analysis of the independent sample t-test, the use of the serious mobile game Jumo the Jumper in primary school classrooms had a significant effect on the vocabulary acquisition of the students compared to traditional instruction. On the other hand, the serious mobile game did not have a significant effect on the students' attitudes toward the EFL course.