Numerous empirical studies confirm the role of self-efficacy in the foreign/second language (L2) learning context. Despite this, few studies have explored the construct of L2 English self-efficacy (L2 self-efficacy) from a longitudinal perspective. This study investigates the developmental features of self-efficacy in English L2 learning by focusing on two research questions: first, whether L2 self-efficacy is a changeable construct, and second, whether L2 self-efficacy growth factors affect L2 English learning outcomes over time. A sample was obtained from the Gyeonggi Education Panel Study (N = 4,501; 1,967 female, 2,084 male) from 63 secondary schools in South Korea. The curve-of-factor model approach using M"plus" 8.4 revealed that L2 self-efficacy slightly increased over three years. Furthermore, the growth of L2 self-efficacy beliefs was positively associated with L2 learning outcome-related variables including L2 class comprehension, L2 class engagement, and L2 achievement, after controlling for the effect of prior attainment and family socioeconomic factors. The findings are discussed in terms of applied language teaching and learning practices, the pedagogical implications in second language acquisition, and further research.