The use of humor has been a controversial research topic in language classrooms. Humor is pervasive; however, the functions of humor in primary English-as-a-foreign-language (EFL) classrooms is under-investigated. To analyze the distinct features of humor, this study explores the specific functions of humor in primary English teaching classrooms in South Korea. The data set included thirty 40-minute English lessons videotaped in 2019 and semi-structured interviews with three teachers. Data were analyzed by identifying humor sequences, transcribing these classroom sessions and interviews, coding recurrent themes, and sorting representative excerpts. As a result, psychological, social, and behavioral functions of humor were conceptualized. The findings showed that humor (i) mitigated learners' anxiety and aroused interest in language learning, (ii) reinforced constructive teacher-learner relationships and enabled camaraderie between learners, and (iii) regulated student behavior or rationalized learners' listening incomprehension. Pedagogical implications are also discussed regarding ways to use and respond to humor in language classrooms.