Ulster Bank is one of the traditional Big Four Irish clearing banks. The Ulster Bank Group was subdivided into two separate legal entities: National Westminster Bank Plc, trading as Ulster Bank (registered in England and Wales and operating in Northern Ireland); and, until April 2023, Ulster Bank Ireland DAC (registered in the Republic of Ireland). Prior to the closure of Ulster Bank in the Republic of Ireland in April 2023, the headquarters of Ulster Bank in the Republic of Ireland were located on George's Quay, Dublin, whilst the headquarters of Ulster Bank Northern Ireland are in Donegall Square East, Belfast, and it maintains a large sector of the financial services in both the UK and the Republic of Ireland.

Established in 1836, Ulster Bank was acquired by the London County and Westminster Bank in 1917. As a wholly owned subsidiary of National Westminster Bank (NatWest), it became part of the Royal Bank of Scotland Group in 2000. RBS Group was renamed NatWest Group in 2020. However, the Ulster Bank brand is used on the island of Ireland. The bank has 146 branches in the Republic of Ireland and 90 in Northern Ireland, with over 1,200 non-charging ATMs. It has over 3,000 employees and over 1.9 million clients.

On 19 February 2021, NatWest Group announced a phased withdrawal of all banking activity and associated services within the Republic of Ireland. On 3 May 2021, the business of Ulster Bank Limited in Northern Ireland was transferred to the parent National Westminster Bank as part of a court-approved Banking Business Transfer Scheme. Ulster Bank ceased its operations in the Republic of Ireland on 21 April 2023. From 2014 to 2023, it had been designated as a Significant Institution under European Banking Supervision and thus directly supervised by the European Central Bank.

Ulster Bank was founded as The Ulster Banking Company in Belfast, Ulster, in 1836, by a breakaway faction of shareholders in the newly formed National Bank of Ireland, which had been founded in 1835, who objected to the latter bank's plan to invest profits from the bank in London rather than in Belfast. The founding directors of the bank were John Heron, Robert Grimshaw, John Currell, who was a linen bleacher from Ballymena, and James Steen, a Belfast pork curer.

Ulster bank opened for business on 1 July 1836 on Waring Street, Belfast. It issued its own banknotes from the outset. Within a year, it opened branches at Antrim, Armagh, Ballymoney, Comber, Downpatrick, Enniskillen, Lurgan, Portadown and Tandragee. In the following two decades, it opened eleven more branches throughout the nine counties of Ulster.

In 1860, Ulster Bank opened a new head office on Waring Street, Belfast, together with branches in Sligo and in Ardee, Co. Louth. In 1862, it opened an office at College Green, Dublin. Over the following twelve years, it opened another 24 branches. It established a limited liability company in 1883, named Ulster Bank Ltd. It opened further branches in Dublin, on Baggot Street and Camden Street, and moved to a new premises on College Green in 1891.

In the early 1900s, Ulster Bank opened branches in Wexford, Dún Laoghaire, Cork, Waterford and Limerick. It was acquired by the London County and Westminster Bank in 1917, but retained its separate identity.

In the 1960s, Ulster Bank opened branches in the suburbs of Belfast and Dublin, together with branches in airports. It introduced ATMs.