This study is to explore a Korean ESL student’s writer identity in the United States. Although previous studies focus on international students’ socialization and identity in American tertiary education, research on their writer identity has been scant. The present study aims to fill this gap by investigating how a Korean international student negotiates her writer identity in the United States. This research uses a qualitative case study that includes five interviews and two classroom observations with the participant as well as interviews with her ESL instructor in a writing course during one semester. The findings indicate that the participant’s writer identity is dynamic, multiple, and transitional over time and context based on her life history in an academic community. In addition, she negotiated her writer identity based on social interaction with an ESL instructor in a written communication. Finally, an alternative way of writing practices outside the classroom was found to contribute to constructing a confident writer identity. This study sheds light on how L2 writers are trained to reflect their own voice in academic writing.