Adequate stakeholder involvement and interactions are essential for effective Integrated Water Resources Management. In Brazil, the decision-making process was decentralized at the river basin committee level, which compromises both government and non-government representatives. However, until now Brazil has struggled to effectively implement IWRM. This paper analyses stakeholders’ perspective regarding the water policy and explores their interactions in the state of São Paulo, Brazil. Using a mixed- methods approach, combining the stakeholder and complexity theories, an exploratory case study was conducted. Data was obtained via an online survey of 206 participants. Results identified different levels of interactions, where stakeholders from the water sector demonstrated higher levels of interactions compared to institutions from other sectors (e.g., environment, energy,) and from the local levels (e.g., city council). Results also indicated the importance of the river basin committees, as they were considered to propitiate adequate participation. They are, however, not enabling adequate interactions. This is because a significant number of stakeholders still do not have formal ways of interaction in their institutions. The analysis led to a set of recommendations to improve water sector stakeholder’s interactions, including interaction with stakeholders outside the water sector. This was considered a key way to seek integration, and, most importantly, to enhance water policy implementation.