The involvement of a native English speaker teacher (NEST, henceforth) in teaching English is a new phenomenon in Sumbawa Besar. SMAN 2 Sumbawa Besar involved a NEST sent by the American Indonesian Exchange Foundation (AMINEF) through the Fulbright English Teaching Assistants (ETA) program. This study aims at describing the roles of a NEST on the students' motivation in learning English at SMAN 2 Sumbawa Besar. The design of this study was descriptive qualitative. The subjects were 27 of the tenth-grade students of SMAN 2 Sumbawa Besar. The following procedures such as observation, documentation, questionnaire, and an interview are used in collecting the data. The results showed that the existence of a NEST on the students' motivation in learning English had a vital implication on the following roles: pedagogical roles, psychological roles, language roles, and cultural knowledge roles.