Intercultural communicative competence (ICC) has been widely acknowledged as a core element of today’s foreign language education. However, even though the importance of intercultural language teaching is commonly recognized among adult learners and at the post-secondary level, teachers of adolescent English learners often find it hard to effectively incorporate culture into English learning because of the lack of an instructional model facilitating their students’ intercultural development and English learning experiences. Therefore, this study aims to investigate whether integrating intercultural learning into an online EFL curriculum can elevate teenage EFL students’ L2 motivation, intercultural communicative competence, and English proficiency. The researchers used a quasi-experimental design by randomly selecting two eighth-grade classes in a secondary school in northern Thailand, with one class designated as the experimental group (N = 31) and the other as the control group (N = 28). The effects of this teaching experiment were then examined using both quantitative and qualitative data. The findings demonstrated that the students in the experimental group showed a greater improvement compared with those of the students in the control group after an 8-week, interculturally embedded English curriculum. The results suggested that ICC is conducive to adolescent EFL students’ intercultural development as well as their English learning motivation and outcome. The applications of ICC-based EFL instruction in similar contexts are discussed.