The company, whose parent group already offers flights to London Heathrow from the east Belfast airport, will add Manchester, Birmingham, Cardiff and East Midlands to its list of destinations from 10 January.
It says the move is aimed at providing "more convenient services" by having all its airlines operate out of the same facility, with up to 48 flights scheduled per week.
"It makes obvious business sense to have the bmi group airlines serving the same Belfast airport," said bmi CEO Wolfgang Prock-Schauer.
"Bmi British Midland International has a long and proud connection with Northern Ireland, serving Belfast for over 26 years."
Brian Ambrose, Chief Executive at Belfast City Airport, described the announcement as "great news".
"Everyone at Belfast City Airport extends a warm welcome to bmibaby," Mr Ambrose said.
"We're delighted that bmibaby recognise the continuing investment in the airport's infrastructure, aimed at improving the passenger experience, which we believe makes Belfast City Airport world class.
"This is great news not only for the 1,500 people employed here but the wider community who rely on the airport's continuing success."
Iceland
Meanwhile Belfast International Airport has announced that tourists will be able to fly between Iceland and Northern Ireland, starting next summer.
The new services, operated by Iceland Express, will open up direct travel channels across the two countries.
It is hoped the move will add revenue to the local economy.
"Our priority is to invest in the provision of new and vital direct access channels," said Uel Hoey, the airport's Business Development Director.
"The aim should be to develop the market, not splinter or replicate services to the detriment of the local economy.
"We are not in the business of simply re-arranging the existing business in the market with the aim of satisfying some short term objectives. We continue to strive to have the broad regional objectives of our business recognised and supported by Northern Ireland's policy-makers."
Flights between Belfast and Reykjavik, Iceland are to commence in summer 2011.