Smartphones compose an integral part of modern life. Besides personal use, mobile phones are being increasingly used in workplaces. Mobile instant messaging (MIM) is a communication technology that features text messaging services and sharing of images, videos, audio, documents, and other media. A key feature of MIM is the creation of groups where individuals can communicate, engage in group discussions, and share knowledge. Knowledge sharing has become an integral part of the organization. It involves social interaction and occurs when employees seek knowledge and others respond by contributing knowledge. The phenomenon of sharing knowledge by MIM applications is common among the employees of the public sector. However, there is a scarcity of literature that investigated the motivations and the risks of using MIM applications for knowledge sharing. This study aims to investigate the motivations and the perceived risks of using MIM applications for knowledge sharing by employees in the public sector. After reviewing the current literature, a research framework has been proposed based mainly on People, Process and Technological (PPT) for the motivations, and Confidentiality, Integrity, and Availability (CIA) for the risks. Then, hypotheses have been developed to assess the significance of the factors in the research framework. A quantitative research approach has been followed using the questionnaire instrument to collect data to verify the hypotheses. The questionnaire was answered by knowledge workers in the public entities in the Sultanate of Oman. SEM-PLS analysis approach was used to analyse the collected data. The findings revealed that people, process, and technological motivations had significant effects on employees' usage of MIM applications for knowledge sharing, whereas no evidence was found supporting employees' perception of risks in using MIM applications for knowledge sharing. This study contributes to the body of knowledge management field by shedding light on the phenomenon of using virtual groups in mobile instant messaging applications to share knowledge within public sector organizations. Additionally, this study contributes to the scientific field by investigating both motivations and risks of using these applications for knowledge sharing in the public sector. Moreover, organizations in the public sector will be better aware of the aspects driving employees' motivations and the status of their perception of the risks of this phenomenon, and consequently, the IT and security policies could be updated considering the findings of this study.