Climate change disrupts the water cycle, jeopardises the security of water, food, and energy systems, and forces states to rethink water management practices to adapt to the new hydrological realities. When states share a water resource, adaptation to climate change proves to be more burdensome, especially if the resource in question is governed by a transboundary water agreement that fails to incorporate flexibility. The focus of this paper is on transboundary water basins with more than two riparian states, where only part of the co-riparian states are party to a transboundary water agreement. The paper discusses whether the recognition of the principle of equitable and reasonable use of water as a principle of customary international law creates an obligation for the party states to such agreements to adapt the terms of their arrangement to climate change. It argues that the principle of equitable and reasonable use of water establishes an obligation of conduct for the states to respect non-parties' riparian rights to a fair share of beneficial uses of water and protect the environment. Therefore, the nature of disruptions caused by climate change may make it imperative for the party states to take all possible measures that allow them to modify their water practices under an agreement to the implications of climate change in order to comply with the principle of equitable and reasonable use of water.