This study attempts to address the “teacher expertise gap” in L2 writing research, using “adaptive expertise” as a specific tool for understanding the notion of writing teacher expertise. Drawing upon language teacher education research and L2 writing research, and using teacher interviews and classroom observation as data sources, the study explores the expertise of three writing teachers from different schools in Hong Kong, as well as the factors that may have contributed to their expertise. The findings of the study show six key components of teacher expertise as demonstrated in the participating teachers, comprising integrated knowledge base about writing and teaching writing; student-centred pedagogy focusing on learner motivation and confidence building; professional visions, self-agency, and reflectivity; leadership in writing innovations; passion as teachers of writing and as writers; and ongoing teacher learning and progressive problem solving as pivotal to writing teacher expertise. The study suggests the rich, dynamic, and contextual nature of writing teacher expertise, which is influenced by teachers’ cognitive engagements, social interactions, and affective experiences in their situated contexts. The paper concludes with implications for the teaching of writing and writing teacher education.