Welsh Water's HelpU social tariff could offer savings of up to £250 a year for its most disadvantaged customers, the company said.
It is backed by the Welsh government and other organisations.
It comes as the water company's half year financial report showed a strong overall performance for the six months leading up to September.
The non-shareholder company - which says all its gains go to customers - said it invested £174m in large projects and essential infrastructure in that period - nearly £1m a day.
It also confirmed that a further £170m will be invested between October 2014 and April 2015.
Chief Executive Chris Jones said: "With all our gains going to customers, we can deliver real value by accelerating investment, keeping bills low and helping those who genuinely struggle to pay their water bills."
The HelpU tariff has been developed with support from the independent consumer watchdog Consumer Council for Water (CCWater), the Welsh government and housing associations group Community Housing Cymru.
It will be targeted at customers spending more than 5% of their income on water bills.
It said average household incomes in the area it serves are almost 13% lower than the rest of England and Wales, with some areas being 20% below the average.

Anthony Tavernor has been restoring Plas Cadnant Hidden Gardens, near Menai Bridge, on Anglesey for 20 years, describing it as his life's project.
But after days of heavy rain, a "tidal wave" of flood water swept through the garden in the early hours of Saturday.
It came as floods caused "chaos" across north Wales.
Former farmer Mr Tavernor has been restoring the 10-acres of garden and buildings at Plas Cadnant since buying the then overgrown 200-acre estate in 1996.
It now attracts visitors - both local and from abroad - and its fans include the Prince of Wales, who Mr Tavernor said had hoped to visit the estate.
But the force of the water from the flooded River Cadnant has now washed away many precious plants, including some rare botanical species.
It also demolished a wall dating back some 200 years, as well stone obelisks, platforms and seats.
"The garden sits down in the valley so a huge amount of water came down from the fields like a tidal wave, devastating everything in its path," said Mr Tavernor.
"I'm just devastated. This has been my life's project - my purpose in life - for the last 20 years and I live and dream it,
"It's everything. I'm so emotionally involved with the garden. We've had so much support from people since we put the pictures on our Facebook page. It's almost like a bereavement."
But he insisted he would carry on his work, despite the setback, and aimed to open as normal for visitors in February.
"I was hoping to have a legacy for the future generations," he added.
"I'm sure a lot of people would abandon it but I'm going to persevere."
Days of rain caused floods which led to roads being closed, trains disrupted and homes evacuated on Boxing Day.
Four flood warnings remain in place across north and west Wales, along with dozens of flood alerts.
More than 100,000 customers in Wales could have help with their water bills thanks to a new tariff being launched.
[NEXT_CONCEPT]
A historical walled garden has been "devastated" by floods which have washed away a 200-year-old wall and rare plants, its owner has said.