Water conflict typically refers to violence or disputes associated with access to, or control of, water resources, or the use of water or water systems as weapons or casualties of conflicts. The term water war is colloquially used in media for some disputes over water, and often is more limited to describing a conflict between countries, states, or groups over the rights to access water resources. The United Nations recognizes that water disputes result from opposing interests of water users, public or private. A wide range of water conflicts appear throughout history, though they are rarely traditional wars waged over water alone. Instead, water has long been a source of tension and one of the causes for conflicts. Water conflicts arise for several reasons, including territorial disputes, a fight for resources, and strategic advantage.

Water conflicts can occur on the intrastate and interstate levels. Interstate conflicts occur between two or more countries that share a transboundary water source, such as a river, sea, or groundwater basin. For example, the Middle East has only 1% of the world's fresh water shared among 5% of the world's population and most of the rivers cross international borders. Intrastate conflicts take place between two or more parties in the same country, such as conflicts between farmers and urban water users.

Most water-related conflicts occur over fresh water because these resources are necessary for basic human needs but can often be scarce or contaminated or poorly allocated among users. Water scarcity worsens water disputes because of competition for potable water, irrigation, electricity generation and other needs. As freshwater is a vital, yet unevenly distributed natural resource, its availability often influences the living and economic conditions of a country or region. The lack of cost-effective water supply options in areas like the Middle East, among other elements of water crises can put severe pressures on all water users, whether corporate, government, or individual, leading to tension, and possibly aggression.

There is a growing number of water conflicts that go unresolved, largely at the sub-national level, and these will become more dangerous as water becomes more scarce, climate changes alter local hydrology, and global population increases. The broad spectrum of water disputes makes them difficult to address, but a wide range of strategies to reduce the risks of such disputes are available. Local and international laws and agreements can help improve sharing of international rivers and aquifers. Improved technology and institutions can both improve water availability and water sharing in water-scarce regions.

Water scarcity has most often led to conflicts at local and regional levels. Water is a vital element for human life, and human activities are closely connected to availability and quality of water. Water is a limited resource. Water conflicts occur because the demand for water resources and potable water can exceed supply, or because control over access and allocation of water may be disputed, or because water management institutions are weak or missing. Elements of a water crisis may put pressures on affected parties to obtain more of a shared water resource, causing diplomatic tension or outright conflict.

Tensions and conflicts over water now occur more frequently at the subnational, rather than the transnational, level. Violence between pastoralists and farmers in sub-Saharan Africa are on the rise. Attacks on civilian water systems during wars that start for other reasons have increased, such as in Yemen, Syria, Iraq, and most recently Ukraine. Water scarcity can also exacerbate conflicts and political tensions which are not directly caused by water. Gradual reductions over time in the quality and/or quantity of fresh water can add to the instability of a region by depleting the health of a population, obstructing economic development, and exacerbating larger conflicts.

Climate change and growing global populations also combine to put new pressures on limited water resources and increase the risk of water conflict.

Over the past 25 years, politicians, academics and journalists have frequently expressed concern that disputes over water would be a source of future wars. Commonly cited quotes include: that of former Egyptian Foreign Minister and former Secretary-General of the United Nations Boutros Boutros-Ghali, who forecast, "The next war in the Middle East will be fought over water, not politics"; his successor at the United Nations, Kofi Annan, who in 2001 said, "Fierce competition for fresh water may well become a source of conflict and wars in the future," and the former Vice President of the World Bank, Ismail Serageldin, who said the wars of the next century will be over water unless significant changes in governance occurred. Moreover, "it is now commonly said that future wars in the Middle East are more likely to be fought over water than over oil," said Lester R. Brown at a previous Stockholm Water Conference.