Teacher collaboration has increasingly been recognised as one of the most impactful models of professional development, particularly when embedded within the everyday practices of schools (e.g., OECD, 2019; Tallman, 2021). Despite this recognition, the concept of teacher collaboration remains inconsistently understood and is often interpreted in divergent ways (e.g., De Jong et al., 2022; Kolleck, 2019; Webs & Holtappels, 2018). Moreover, the existing literature has largely focused on collaboration within Western education systems, with limited empirical attention to its enactment in non-Western contexts. This study addresses these gaps by exploring teacher collaboration within the Indonesian school system. Specifically, it investigates how collaboration is conceptualised and practised and identifies the factors that influence teachers’ engagement in collaborative activity. A multiple case study design was employed to examine the socially mediated and tool-driven nature of teacher collaboration in three secondary schools in Indonesia. Fifteen participants comprising teachers and principals from the three schools were purposively selected. Data were collected through semi-structured interviews, document analysis, and field observations. The third generation of Cultural-Historical Activity Theory (Engeström, 2001) was used as both a conceptual and analytical framework to guide data collection and analysis. The findings reveal that teacher collaboration in Indonesia is a multifaceted and dynamic process shaped by the interplay between institutional structures, professional goals, and socio-cultural values. This study proposes four interconnected conceptualisations of teacher collaboration: (1) individual collaboration, where teachers engage independently in shared activities with a strong sense of agency; (2) social connection, where collegial support and interpersonal relationships form the basis of collaboration; (3) joint work, which involves collective engagement in instructional planning and assessment; and (4) school agenda collaboration, shaped by top-down mandates aligned with institutional priorities. The study also shows that teachers engage in both formal and informal collaboration, shaped by five key dimensions: temporal structure, institutional scope, agency and mandate, collaborative purpose, and disciplinary focus. Collaboration in Indonesia remains largely subject-specific, limiting opportunities for interdisciplinary dialogue. Moreover, the findings highlight how structural, cultural, and contextual factors such as government policies, school leadership, curriculum design, workload, and school culture mediate the conditions under which collaboration occurs. These factors can act as both enablers and constraints, influencing the extent and nature of teachers’ collaborative engagement. Collaboration is also perceived as a moral and ethical obligation, reinforcing a shared sense of professional purpose and aligning teacher learning with religious values. These findings carry important implications for research, policy, and practice. The study calls for policies that balance structured, mandated collaboration with opportunities for teacher agency. It also underscores the value of informal and spontaneous collaboration, and highlights the need for school-level strategies that promote more meaningful, interdisciplinary, and reflective professional learning.