This study investigated novice teachers’ perceptions of their professional agency during the initial years of their work in schools. The research questions were: (i) How do novice teachers perceive their professional agency within their work, and what do they see as the main restrictions and resources affecting that agency? (ii) How do novice teachers perceive their professional agency in the construction and renegotiation of their professional identities, and what do they see as the main restrictions and resources affecting their sense of agency? In theoretical terms, we adhere to a subject-centered sociocultural approach. This implies understanding subjects as active agents from a developmental perspective, with attention also given to professional identities. The subjects of this study were 13 qualified primary-level class teachers who had worked from one to five years as class teachers in Finnish primary schools. Data from open-ended interviews with the novice teachers were analyzed using thematic analysis. The findings indicated that all the novice teachers perceived strong agency in their opportunities to apply and develop pedagogical practices within the classroom. However, they had a much weaker sense of agency as regards the social management of the classroom. When the teachers entered into the practical school context, most of them became aware that they would have to renegotiate their professional identities. This involved re-assessing their professional ideals and ethical standards. The major perceived constraint emerged from their lack of competence in providing support for the children’s psychological well-being. The role of the school principal was seen crucial for the novice teachers’ work in the schools: the principal was seen as acting as a resource but also as a constraint on the teacher’s sense of professional agency, both at the individual and school levels. In addition, close collaboration and support from other teachers in difficult everyday situations was seen as crucial for the teachers’ survival in the practical school context. The study provides insights into the challenges and constraints affecting novice teachers’ sense of professional agency, and on the resources and support that they need in their first years at work.