The move reflects the fall in wholesale gas prices, which have been falling since the end of last year and are now about £1.10 per therm.
Firmus Energy is one of two gas suppliers in Northern Ireland.
The other - SSE Airtricity - will not be reducing its price in April, the Utility Regulator has confirmed.
Firmus Energy has about 110,000 customers across Northern Ireland and cut its prices by about a fifth back in January.
Customers in the Ten Towns network area, which includes Antrim, Armagh, Ballymena, Coleraine, Craigavon, Londonderry and Newry, will see prices fall by 18.84%.
Those in Greater Belfast, which includes Lisburn, Bangor, Larne, Comber, Newtownabbey and Carrickfergus, will see prices fall by 22.43% from April.
That will save customers in the Ten Towns £335 a year and those in Greater Belfast £433 a year on average, according to Firmus Energy.
Its chief executive Niall Martindale said: "We previously committed to our customers that as soon as market conditions allowed us to lower our prices we would do so.
"We recognise the wider cost-of-living challenges within the communities we serve and at Firmus, we are working hard to make positive change for our 110,000 loyal and valued customers."
Firmus Energyʼs price decrease is a result of a review undertaken in January by the company and the Utility Regulator, which regulates the industry in Northern Ireland.
Will SSE Airtricity reduce its gas price?
There will be no change to the price of gas from SSE Airtricity in April, the Utility Regulator has concluded after reviewing the companyʼs tariff.
SSE Airtricity has 186,000 customers in Northern Ireland.
Utility Regulator chief executive Kevin Shiels said the review of SSE Airtricity found that a price drop was not required at present.
"Whilst the review showed that falling wholesale gas prices have placed downward pressure on the tariff, this was offset by increases in other costs, such as the rising costs of distributing gas," he said.
"We will continue to monitor SSE Airtricityʼs regulated gas tariff and ensure any falls in costs are passed through to customers at the earliest opportunity."
In a statement to BBC News NI, SSE Airtricity said: "We note that recent tariff reviews have brought gas tariffs in closer alignment across suppliers in the market.
"We understand that energy prices are a primary concern for our customers and we encourage any customer who may be experiencing difficulty to contact us and we will work with them to find a solution."
What else might affect gas prices?
The final effect of those announcements on customersʼ bills will be determined by what the UK government does in relation to its Energy Price Guarantee.
The government is subsidising energy bills through the Energy Price Guarantee, which reduces the amount people can be charged for each unit of gas by 3.893p/kwh until the end of March.
The BBC understands that the Chancellor Jeremy Hunt is expected to extend the Energy Price Guarantee for another three months.
An announcement is expected to be made on that in the chancellorʼs budget on 15 March.
What is happening to energy bills?
Help with energy bills set to be extended in April
Mr Shiels from the Utility Regulator said any change in the level of support provided through the Energy Price Guarantee scheme "would impact the final tariff customers pay".
"If the Energy Price Guarantee discount changes, Firmus Energy will adjust their tariffs accordingly and advise their customers," he added.
The Consumer Council for Northern Ireland warned that if the government was to reduce the support in the Energy Price Guarantee scheme it would mean that Firmus Energy customersʼ bills would not fall by as much as announced on Tuesday.
Raymond Gormley from the Consumer Council said he anticipated that the government would reduce the support it would offer.
He said that gas bills were higher than last year and advised anyone struggling to pay to contact their supplier for help.