Based on the motivation theories, we postulated that interest in learning languages influences the self-efficacy beliefs of students about their capabilities as second/foreign language (L2) English learners. This study examined the longitudinal causal relation between affective interest and cognitive self-efficacy in the L2 motivational context using the Gyeonggi Education Panel Study (GEPS) data from South Korea. A secondary school student sample from 2015–2020 GEPS over six years comprised 6,314 students (3,189 males and 3,125 females). Using a parallel growth model (PPM) with Mplus 8.4, the results indicated that the initial level and growth rate of L2 English interest positively predicted L2 English self-efficacy. Subsequently, the initial level of L2 English interest exhibited a negative cross-effect on the growth rate of L2 English self-efficacy. Finally, the sequential causal effect of L2 English interest on self-efficacy was found across the six-year period. The findings are discussed in terms of the pedagogical implications in English learning and teaching practices and further research.