Aim/Purpose: The study aims to explore the dimensions of identities in relation to an online community of practice (CoP) and how the dimensions of identities influence the way teachers behave on their online CoP. Background: One of the emerging approaches for teachers’ professional development is through a form of community of practice, through which teachers learn through collaboration and active learning. In line with the progression in technology, online communities of practice have been widely accepted as one of the possible approaches for teacher professional development that can enhance the opportunity for collaboration. Even though online CoPs provide a better platform for collaboration and sharing best practices among teachers, some issues lead to a failure of any online CoPs. Day, Sammons, Stobart, Kington, & Gu (2007) stress the importance of understanding the aspects of identities and their impact on how teachers perform and commit to any activities and that an understanding of teachers’ identities is central to any analysis of teachers’ effectiveness, work, and lives. Previous research, however, studied the aspects of identities in the perspectives of the development of their identities as teachers in the context of their daily interaction with significant others in face-to-face mode. However, there has been very little research that has focused on teachers’ identities in relation to their participation in online communities. The extent to which their identities influenced the way they interact, engage, and contribute to their online CoP is still debatable, although it was profoundly stated that identities play a great role in shaping teachers’ behavior in their offline CoPs. Taking this into account, this study aims to identify the dimensions of identities in an online CoP setting and how these identities influence their capacity to involve themselves in online sharing through communities of practice (CoPs). Methodology: This research employed a case study approach which involved 16 teachers from six high performing secondary schools. The selection of the participants was made through purposive sampling. Data was generated through in-depth one-to-one interviews. Data analysis was conducted using thematic analysis through which the emerging themes were carefully identified. Contribution: The study has successfully identified the dimensions of identities in relation to teachers’ participation in an online CoP, which adds to the current body of literature. The result of the study also illustrates how these dimensions of identities interrelated to each other that led to the teachers’ level of participation in an online CoP. Having an in-depth understanding about identities also would provide a better understanding of why the members reacted the way they did and, and how the dimension of identities plays a role in this. Findings: The result of the analysis indicates four main dimensions of identities, i.e., personal identities, professional identities, learner identities and member of the community’s identities. These dimensions were found to influence each other. Overall, there are seven factors seen as ‘immediate’ causes leading to the final outcome (participation in online CoPs), i.e., beliefs in the benefits of informal sharing activities, perceived importance of online sharing activities, perceived role in community, willingness to initiate discussions, willingness to respond, acceptance towards others’ comments, and beliefs in the benefits of online communities. Personal identities affected not only their jobs as teachers but also influenced their commitment towards their participation in the online CoPs in this project. Their prior knowledge and experience influenced teachers’ perceived competency in using online sharing applications. Their prior experience also impacted the way they perceived the benefits of online activities (teachers’ identities as learners) and their attitudes towards them. The findings indicate that different individuals had different sharing preferences, and the differences were partly driven by how they conceived of professional development as well as how they perceived themselves professionally. Recommendations for Practitioners: This study also indicates that to ensure the success of any online professional development for teachers, it is essential to take into consideration the aspect of endorsement by senior management, e.g., principals or coordinators from a district or state level. It is also critical for stakeholders to understand the working culture of teachers and their conception of professional development to ensure any new policies is in line with teachers’ identities. Recommendation for Researchers: The analysis in this study was developed by exploring the reasons behind the teacher’s behaviors. In the future, it will be more meaningful for new researchers to consider the dimensions of identities when they develop any online CoP. Future Research: This study was conducted using a qualitative approach. The emerging dimensions of identities can be used by future researchers as a basis to do quantitative research that covers a larger sample size, through which a generalization can be made. A causal network that was developed in this study can be tested using inferential statistics.