In today's increasingly diverse educational landscapes, the professional lives of first language (L1) teachers are undergoing significant transformations. This chapter explores the concept of ‘transprofessional identity’ as it pertains to the changing self-definition of L1 Chinese language teachers in increasingly multilingual, multicultural school settings. While prior research extensively examined the translingual identity of second language (L2) teachers in multilingual contexts, the unique transprofessional experiences of L1 teachers have been overlooked. This research fills this gap by analyzing the narratives of two L1 Chinese language teachers' professional trajectories. Ms. Au, a long-standing L1 teacher in a Hong Kong public school, faced the challenge of instructing South Asian ethnic minority students due to demographic shifts; Mr. Bao transitioned from teaching L1 to local Hong Kong students to L2 to multicultural learners in an international school. This chapter delves into the complexities of their transprofessional identity negotiation, encompassing pedagogical approaches, student diversity, and evolving institutional contexts. The analysis reveals the multifaceted, boundary-crossing negotiations undertaken by the two teachers. This chapter challenges the conventional notion of language teacher professionalism, which rigidly segregates L1 and L2 teaching as distinct subject areas. Copyright © 2024 selection and editorial matter, Sin Manw Sophia Lam, John Chi-Kin Lee and Chung Mou Si; individual chapters, the contributors.