Increased interest in open government data initiatives stems from long debates about state modernization. This study investigates the challenges imposed on public administration regarding data publishing from an open data perspective to propose a maturity diagnosis model for open data portals, aiming at the provision of in-depth, consistent, efficient, and transparent government information. The research comprised three phases. Firstly, we conducted a pre-test on open data portals from the United States, the United Kingdom, Brazil and Ireland, motivated by studies on the history of the open data movement as well as the current panorama of initiatives, evolution stages and challenges, which defined the fundamental analytical dimensions for the proposed maturity model. Secondly, we developed a maturity diagnosis model for open government data portals. Finally, the model was applied and validated on the pre-tested portals. Results from the diagnosis instrument can guide public administration in realizing a more efficient and responsible data governance, benefiting the government, the open data movement and civil society.