The average household’s water and sewerage bill will drop by about £17 a year in England and Wales.
Water UK, the industry trade body, said the average annual bill will fall by 4% to £396.60 from 1 April.
Its chief executive, Christine McGourty, said the water industry is “committed to giving customers good value for money”.
The regulator Ofwat said the drop was down to the fact it had “demanded greater efficiency” from firms.
Changes to bills will vary from customer to customer and depend on their supplier.
Water and sewerage bills by supplier
2020-21 average for England and Wales
The commitment comes at the start of the next five-year “business cycle” for water companies.
Water UK said that there would also be more help for vulnerable customers between 2020 and 2025.
It said firms plan to double the number of people getting help with their bills every year, up from 760,000 customers now to at least 1.4 million by 2025.
Measures to assist them could include introducing social tariffs, discounts for those on lower incomes or receiving benefits, or assisting charities that provide help.
The water regulator Ofwat told suppliers in December that they would have to cut the average customer bill by £50 over the next five years.
There has been recent criticism of water companies over high profile pollution incidents as well as leaks, water quality and high bills.
An Ofwat spokesperson said: “We continue to push companies to deliver improved services for customers, the environment and resilience for generations to come while making sure that bills are fair.”
They added: “Today’s announcement has been secured because we have demanded greater efficiency, passing through lower financing costs and promoting more innovation.”
At the same time, it is forcing firms to restrict investor payouts and to invest billions of pounds to improve their performance and reduce leaks in their systems.
Ofwat is the government regulator, which oversees the privatised water market in England and Wales. It monitors the market to see if it needs to intervene to protect customers and to set limits on the price they are asked to pay.
Scotland has its own separate regulator, the Water Industry Commission for Scotland.
The Consumer Council for Water told consumers to “take full advantage” of the reduction in the average water and sewerage bill.
Andy White, its senior policy manager, said: “Many households don’t feel they get a fair deal from their water company but over the next five years customers are set to receive more for their money.
“There are still millions of households who could tap into savings by switching to a meter or cut their bills if they’re on a low income by signing up to their company’s social tariff.”