Emerald Airlines, which operates Aer Lingus Regional services, it to start two new routes to Cardiff and Southampton from its Belfast City Airport base from July 7.
The airline already flies to Birmingham, Edinburgh, Leeds Bradford, Glasgow, Exeter, and Manchester from Belfast.
With the addition of Cardiff and Southampton, Aer Lingus Regional will now operate routes to eight of the 21 destinations Belfast City Airport will fly to this summer.
"As we near the peak summer months, the launch of our Cardiff and Southampton services will provide those travelling from Belfast with more choice when planning their holidays," Ciarán Smith, Head of Commercial at Emerald Airlines, said.
Ellie McGimpsey, Aviation Development Manager at Belfast City Airport, said that Aer Lingus Regional's base at Belfast City Airport has been operational for just over three months.
She said the airport is thrilled to already see expansion in both the number of routes served by the airline and the frequency of its services.
The Aer Lingus Regional flights will be operated by the ATR72-600 plane, the latest generation of turboprop aircraft combining unmatched environmental and economic performance.