Perceived English reading self-efficacy is one of the essential factors that facilitate development of English language ability as it motivates learners to read and learn. The present study aimed to investigate the outcomes of out-of-class extensive reading to enhance perceived reading self-efficacy of underserved student and to explore underserved students’ attitudes towards out-of-class extensive reading. In this study, a mixed-method research design, with a particular focus on the use of narrative inquiry, was employed, and the study participants consisted of five lower secondary students in a slum community in Bangkok, Thailand. The purposively selected participants were between 14 and 15 years old, and all of them were considered underserved students. Data were collected by means of the pre- and post-perceived reading self-efficacy questionnaires, students’ log, teacher’s diary, interview protocols and attitude questionnaire. The findings revealed that out-of-class extensive reading could enhance underserved students’ perceived reading self-efficacy and the underserved students had a positive attitude towards out-of-class extensive reading as it suited specific characteristics and learning conditions of underserved students and motivated them to read. Based on such findings, it could be concluded that out-of-class extensive reading could be utilized to enhance perceived reading self-efficacy, which, in turn, could help develop reading ability of underserved students.