Npower is raising energy bills by around £64 a year for a million customers this year.
The average 5.3% dual fuel price hike comes into effect on June 17 and follows earlier rises announced last month by its "Big Six" rivals.
British Gas is increasing prices by 5.5% from May 29, while Scottish Power is raising prices by 5.5% on June 1.
Npower blamed increases wholesale energy costs and government policy changes for the rise.
The 5.3% average price hike is made up of a 4.4% rise in gas prices and a 6.2% increase in electricity.
It will see a typical dual fuel gas and electricity annual bill climb to £1,230.