First Trust Bank is to close 15 branches in the North.
The bank said the move followed an 18-month strategic review to ensure sustainability In the face of changing customer behaviour habits.
As well as the region-wide closures, which will halve the number of branches, the bank said it planned a £10 million investment strategy for personal and business customers, including five new business centres.
The bank, which is part of the AIB group, has also agreed a partnership with the Post Office to enable customers to conduct their everyday banking transactions in Post Offices.
Head of First Trust Bank Des Moore said: "Since 2010 we have seen almost a 40% decline in branch usage coupled with a 117% increase in mobile transactions over the past two years and we have had to make the difficult decision to close a number of branches.
"We will be working hard to minimise the impact on the customers and staff concerned."
He added: "We recognise that the pattern of how customers are managing their finances is rapidly evolving in Northern Ireland as it is across Europe.
"Customers are turning to online, telephone and mobile banking and it is essential that we respond.
"As a result, our transformation programme focuses on building a sustainable business model which allows us to support our customers and the wider Northern Ireland economy in the long term."
The bank said its existing voluntary severance programme would be extended to impacted staff.
General Secretary of the Financial Services Union, Larry Broderick, said the scale of the branch closures was "a serious blow for staff, customers and the Northern Ireland economy".
"While staff were aware of an ongoing restructuring plan at FTB the scale of closures announced today has come as a shock to staff," he said.
"Great teams in bank branches, many who have served their community for many years, now face the prospect closure and relocation. It is a major blow."
He said a pay and job security deal accepted by staff in AIB Group would ensure that there would be no compulsory redundancies.
"While opposing the scale of the closures, the Financial Services Union will work hard over the coming months to ensure that commitments made to staff are upheld and that staff can continue to contribute to the bank and the wider Northern Ireland economy," he said.
"In particular, we will ensure that any staff departures will be on a voluntary basis. This is a position that the union has fought hard to achieve and is in place until 2019."