The increase will affect customers in the Ten Towns and Greater Belfast areas.
The firm said the decision was "unavoidable".
From the 05 April, Firmus Energy customers in the Ten Towns network area will see a price increase of 6.8%. Tariff prices in the Greater Belfast area will increase by 8.8% and will take effect from the 16 April.
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Michael Scott, Managing Director of Firmus Energy, said: "We work hard to keep our tariffs as low as possible, to offer price stability for customers and give them confidence in natural gas.
"The decision to raise our tariffs is regrettable and never easy, however in this instance the cost of bringing natural gas to our customers has increased and our price rise has been unavoidable.
"Our pricing team has engaged extensively with the Utility Regulator, the Department for the Economy and the Consumer Council for Northern Ireland in relation to our regulated tariff in the Ten Towns licensed area.
"We keep our tariffs under continual review to ensure we are bringing the best possible prices to our customers."
(CD/LM)
