Although the role of the institutions is to reduce uncertainty, a weak institutional environment can have the opposite effect, increasing uncertainty for a multinational company (MNC). Therefore, MNCs adapt their strategies in a weak institutional environment. However, according to co-evolutionary theory, they also can influence those institutions. Our research focuses on how MNCs adapt their strategy to uncertainty caused by weak institutions and how they can build influence them. We conducted an inductive multiple case study of seven companies, leaders in their global and national markets, in four industries in Brazil: automotive, beverage, cosmetics, and tobacco. Our findings suggest that high levels of institutional uncertainty not only affects MNCs directly, but that it also serves as a source of relational uncertainty by affecting their suppliers and markets. They also reveal how greater uncertainty can result in internal resource improvement through an exploitation strategy, causing an MNC to have a major influence over its partners and their institutional environments. Finally, they show how market composition can determine channels of influence over institutions, directly by an MNC or indirectly by industry associations. Our study contributes to understanding the role of weak institutions as a source of uncertainty that influences operational strategy definition.