Ulsterbus is a public transport operator in Northern Ireland and operates bus services outside Belfast. It is part of Translink, the brand name for the subsidiary operating companies of the Northern Ireland Transport Holding Company, which also includes Northern Ireland Railways and Metro Belfast.

Ulsterbus was founded in 1967 with the creation of the Northern Ireland Transport Holding Company, replacing the former Ulster Transport Authority. The founding of Ulsterbus coincided with the beginnings of The Troubles, a conflict which intensified throughout the 1970s and 1980s and often saw Ulsterbus buses and employees caught in the crossfire. Drivers would often face hijackings, assaults and robberies while operating Ulsterbus services, their buses being turned into burning barricades or occasionally being bombed. Four Ulsterbus employees were killed in the 1972 Bloody Friday bombings when an IRA bomb exploded at the Oxford Street bus station. In total, 17 employees from both Ulsterbus and Citybus were killed during The Troubles, with 1,484 buses in total being maliciously destroyed from 1964 to the signing of the Good Friday Agreement in 1998.

Werner Heubeck was the first managing director of both Ulsterbus and Citybus, managing both companies throughout The Troubles from his appointment in 1965 until his retirement in 1988. A former Luftwaffe conscript, Heubeck became known for carrying bombs off buses and for making Northern Ireland's buses profitable and running to schedule despite the security situation, and received both an OBE in 1977 and a CBE on his retirement in 1988.

The Northern Ireland Transport Holding Company was rebranded to Translink in 1996. Ulsterbus and Citybus, as well as Northern Ireland's rail services, were integrated into Translink and were subsequently rebranded. Translink have built and refurbished a number of bus stations served by Ulsterbus and have invested in a fully low-floor bus fleet, as well as introducing brands such as the Foyle Metro.

Ulsterbus is responsible for most of the province-wide bus services in Northern Ireland. It operates 1,100 buses and twenty-two bus stations, several of which, such as those at Belfast Grand Central and Bangor, form integrated transport interchanges with Northern Ireland Railways stations.

Ulsterbus, as part of Translink, is charged with transporting over 55,000 children per day across Northern Ireland to school. Every July and August, around 250 vehicles are usually de-taxed. This is because not as many are needed for service due to schools finishing for summer holidays.^{[citation needed]}

Goldliner is the name given to the inter-city express bus services operated by Ulsterbus. Originally launched as the 'Northern Ireland Express' in the 1970s, these services were rebranded to 'Goldline' in 1990, with the Belfast-Derry express service relaunched as the 'Maiden City Flyer' with eight new coaches in 1991. This service has seen major expansion over the years: Belfast-Derry 212 service went from a thrice daily service in 1990 to half-hourly frequency, and many services have been expanded and introduced. In 2021, these services were rebranded again to 'Goldliner' in line with the wider rebranding of Translink.

Goldliner services are primarily operated from Belfast to major destinations in Northern Ireland, as well as 'Xpress' branded cross-border Goldline Express Services X1 (previously service 200) to Dublin (via Dublin Airport). This service was worked jointly with Bus Éireann's service X1 (previously service 001) until December 2020, after which Bus Éireann withdrew their X1. There is also a X2 operating express between Belfast and Dublin Airport without any intermediate stops between them.