This article examines the sociocultural shaping of "teacher agency" by focusing on how various conditions "mediate" teachers' perceived professional space. Agency is understood from a "sociocultural" perspective as the mediated capacity to act, which is achieved and exercised "in" a professional space, and, consequently, shaped by the "perceptions" of this space. Based on thematic and interpretative analysis of interviews with Norwegian L1 upper secondary teachers, the article identifies several conditions that mediate the teachers' perceived professional space: the exam, the curriculum, accountability demands, school leadership, colleagues, students, learning materials, and subject traditions and purposes. Two aspects of mediation are identified, what we respectively term "extension" and "remodelling." Extension conveys how mediating conditions provide resources or constraints that contribute to "expanding or narrowing" the teachers' perceived professional space. Remodelling, however, conveys how mediating conditions "qualitatively transform" the perceived professional space. Findings highlight the subject as a resource for teachers, and teacher agency as socioculturally shaped by the perceived size and character of professional space.