The group is to increase typical annual dual fuel bills by 5.3%, or £64, across all payment types from June 17.
It is made up of an average rise of 4.4% on gas and 6.2% on electricity.
Npower blamed increases in policy and wholesale energy costs for the hike, which comes weeks after EDF and British Gas also raised prices.
Simon Stacey, managing director, domestic markets at Npower, said: “Announcing this price change today isn’t a decision we’ve taken lightly.
“The costs all large and medium energy suppliers are facing - particularly wholesale and policy costs which are largely outside our control - have unfortunately been on the rise for some time and we need to reflect these in our prices.
“While existing customers who are currently on a fixed deal, have a prepayment meter or on the Safeguard tariff are protected from these increases, we encourage any customer who is struggling with their energy bills to contact us.”
More than 60% of Npower’s customers will not face an increase, the company stressed.
Energy firms across the board have been facing pressure for some time over the way they treat customers, with controversy over pricing and the use of standard variable tariffs, which have been deemed poor value for consumers.
The Government also plans to impose a price cap on costly energy products.
Victoria Arrington, spokesperson for comparison site energyhelpline, commented:
“The most expensive standard tariff just got more expensive, going up by £64 on average.
“This also means five out of the Big Six have had price rises in 2018. No one should pay such high prices for their energy.
“Switching suppliers has the potential to be a very effective way of bringing down bills.”