Information technology and systems (IST) projects are fundamental for the development of contemporary organizations, and public institutions are no exception to this reality. Given the importance and complexity inherent to these projects, project management should be conducted with full awareness of the factors that influence success. Several works address the success factors of IST projects; however, the related literature focused on project management in the public sector is scarce. This paper presents a model of success factors for IST projects in public institutions resulting from a literature review, a case study of six Portuguese central government and local government organizations, and a questionnaire-based survey. From the set of factors identified, the results highlight those related to the project's scope and the project manager and team.