Emotional engagement has surfaced as a vital psychological category affecting second language learning, particularly in learner-focussed and interdisciplinary settings. Integrated STEM disciplines (science, technology, engineering, and mathematics) are becoming increasingly popular in English as a Second Language (ESL), particularly as a pedagogical approach that encourages meaningful use of language through real-world problem-solving and inquiry-based tasks. Existing studies exploring cognitive and linguistic outcomes of STEM-integrated ESL learning are plentiful; limited attention, however, has been paid to the psychological factor, emotional engagement in similar contexts. This conceptual paper argues that emotional engagement serves as a mediating psychological English-as-a-second language (ESL) construct between learning integrated science, technology, engineering and mathematics (STEM). Using educational psychology theories, in particular emotional engagement theory, control-value theory of achievement emotions and sociocultural approaches to learning, this paper presents a conceptual framework designed to connect STEM-oriented instructional approaches with learners’ emotional engagement and ESL learning outcomes. It argues that STEM tasks that facilitate emotional engagement can generate positive emotions including enjoyment, interest and confidence which in turn increase ESL learners’ willingness to communicate, language use and sustained engagement with ESL learning. Pedagogical, curriculum design and future empirical research implications are presented.