The purpose of the present analysis was to investigate the impact of altruistic teaching, i.e., teaching with self-less motives (a concept in positive psychology), on the students' task engagement in English essay writing. To this end, the present study adopted two designs, i.e., an experimental (comparison group, pretest-posttest) design to investigate if the learners' altruistic teaching affects their task engagement statistically significantly in the short and long run by using a task engagement questionnaire, and a mixed-methods sequential explanatory design to explore how it affects their task engagement by using reflective frames and semi-structured interviews. The participants were 122 Iranian EFL learners with an intermediate level of English language proficiency. The results of quantitative and qualitative analyses showed that altruistic teaching enhances the students’ task engagement in the short and long run by creating in them a sense of enjoyment, self-verification, responsibility, bonding and group learning, less anxiety, self-confidence, progress in learning, finding out about strengths and weaknesses, and more enthusiasm to learn. The paper ends with theoretical and pedagogical implications for research and practice in second language acquisition (SLA).