The forum, held every three years, will address growing water scarcity, the risk of conflict as countries squabble over rivers, lakes and aquifers, and how to provide clean water and sanitation to billions of people.
It has attracted thousands of activists, scientists, entrepreneurs, mayors, parliamentarians and others invited to promote ideas about water management.
The conference is also a meeting place for companies involved in the multi-billion water industry.
Between $92.4bn and $148bn dollars are needed annually to build and maintain water supply systems, sanitation and irrigation, according to the third World Water Development Report.
How this investment is attained and the accountability of corporations in water are among the questions to be discussed at the seven-day conference.
"Good water law"
Loic Fauchon, president of the World Water Council, holding the conference said: "[We are] responsible for the aggressions perpetrated against water, responsible for the current climate changes which come on top of the global changes, responsible for the tensions which reduce the availability of freshwater masses so indispensable to the survival of humanity."
"Mark Smith of the green group, the International Union for Conservation of Nature (IUCN), said: "In many regions, water scarcity and pollution are increasingly putting human wellbeing at risk."
"We have to organise ourselves to use water more sustainably. We need systems for governing water based on a balance of policy and good water law."
The UN will release a report on Monday warning of the threat to global water supplies from factors including population growth and climate change.
The world's population, currently more than 6.5 billion, is expected to rise to nine billion by mid-century, placing further demands on water supplies already strained.
The number of people living under severe water stress is expected to rise to 3.9 billion by 2030, amounting to nearly half the world's population, the Organisation for Economic Co-operation and Development (OECD) has said.
The forum organisers deny they represent any special interest group.