A key characteristic of Smart Cities is the ability to improve decision making by combining the maximum possible information provided by sensors or people. This paper presents a Blockchain-based reputation system for allowing citizens to assess the quality of information shared by other participating citizens. Citizens have the possibility to confirm or refute information posted by others, and the reputation of participants has a role in the final credibility of the information shared. Individual reputation is used as a “currency”, citizens bet with their reputation when giving an opinion, and get some reputation back when they are considered to be right. The reputation system is supervised by municipality stakeholders, which have the role of giving a final decision on the quality of the information and, consequently, in the update of the reputations associated with it. Stakeholders use Blockchain technology as a natural solution to spread their power regarding reputation management, ensuring a trustworthy dissemination of information, for instance, when reporting accidents.