We presented one new coach's development during a K-12 STEM integration initiative in order to explore how coaches develop professional identities for coaching. Madison, a former mathematics educator, partnered with science teachers for 1 year in order to co-develop and implement STEM curriculum. Transcripts of coaching conversations, written reflection data, assignments from a graduate-level coaching course, and interviews were analyzed using three-phased qualitative methods. Findings suggest that a new coach approximated practice similar to new teachers; relied on existing professional identities when constructing new coaching identities; and the local, global, and institutional affinity groups for STEM coaching both informed and disrupted coaching identity development. The process by which new coaches develop contextually dependent and discipline-specific identities for coaching has implications for the success of STEM coaching efforts.