The potential of using augmented reality (AR) to create constructive learning opportunities in second language classrooms has been widely acknowledged. This understanding is based on the claim that ubiquitous learning paradigms, like AR-based instruction, motivate learners to engage in second language learning. To investigate the engagement and motivation of English as a foreign language learners in non-AR and AR-based vocabulary learning conditions, we performed a series of t-tests on a sample of third-, fourth-, fifth-, and sixth-grade students (n = 38, female = 18 students, mean age = 10.4 years old) whose engagement and motivation were assessed using the four scales of flow state (control, attention focus, curiosity, and internal interest). Moreover, a focus group interview was conducted on five students who were engaged in the AR-based instruction. The results of the flow questionnaire and focus group interview indicated that the AR-based instruction fostered higher interest and curiosity among young learners as compared to the non-AR-based instruction. These findings suggest that AR-based interventions designed to enhance young learners’ motivation to learn English as a foreign language can be beneficial.