This paper argues that undergraduate peer-to-peer instruction in STEM writing provides valuable insights into the relationship between writing and disciplinary identity. Drawing on observation and interview data from a writing center staffed by undergraduate STEM students, I argue that STEM writing tutors construct disciplinary identities by drawing on coursework and extracurricular writing experiences that contribute to rhetorical knowledge. Tutors then leverage this knowledge and experience within tutoring sessions by engaging in explicit genre instruction and disciplinary socialization.