The number of public and private organizations of all sizes that have developed an Information Systems/Information Technology (IS/IT) strategic planning process is gradually increasing in Brazil. As the demand for new methodologies grows, models and processes that help these organizations understand the benefits of using these mechanisms to improve their businesses become necessary to evaluate the maturity level of their organizational processes, especially those related to IS/IT. This paper presents a model to assess the maturity level of the IS/IT strategic planning of Brazilian governmental organizations, named MMPE-SI/TI (Gov). It was designed in accordance with the main models and standards used for national and international definition and assessment of processes and consists of a reference model (RM), an assessment method (AM) and a database of best practices (DBP) set to assist in the development of the organizations improvement plan. The definition of the model involved several steps, including: systematic review, conducting exploratory case studies, organizing a set of best practices, validation with experts from the IS/IT area, analyzing and interpreting data and improvements recommendation. The MMPE-SI/TI (Gov) model has five maturity levels, six capacity levels, 16 processes and 124 best practices for IS/IT strategic planning directed to Brazilian governmental organizations.