Rathlin Island (Irish: Reachlainn, pronounced [ˈɾˠaxlən̠ʲ]; Local Irish dialect: Reachraidh, Irish pronunciation: [ˈɾˠaxɾˠi]; Scots: Racherie) is an island and civil parish off the coast of County Antrim (of which it is part) in Northern Ireland. It is Northern Ireland's northernmost point. As of the 2021 Census there were 141 people living on the island.

Rathlin is the only inhabited offshore island of Northern Ireland, with a steadily growing population of approximately 150 people, and is the most northerly inhabited island off the coast of Northern Ireland. The reverse-L-shaped Rathlin Island is four miles (six kilometres) from east to west, and 2+1⁄2 miles (4 kilometres) from north to south.

The highest point on the island is Slieveard, 134 metres (440 feet) above sea level. Rathlin is 15+1⁄2 nautical miles (29 kilometres) from the Mull of Kintyre, the southern tip of Scotland's Kintyre peninsula. It is part of the Causeway Coast and Glens council area, and is represented by the Rathlin Development & Community Association.

Rathlin is part of the traditional barony of Cary (around the town of Ballycastle), and of Causeway Coast and Glens Borough Council. The island constitutes a civil parish and is subdivided into 22 townlands:

Rathlin Island is labelled as The_Glens_B1 Data Zone, according to the Northern Ireland Statistics and Research Agency (NISRA). As of the 2021 Census it is the smallest Data Zone. On Census day (21 March 2021) there were 141 people living in Rathlin Island. Of these:

The Irish language (also known locally as Gaelic) was spoken on Rathlin Island for most of the last 2,500 years, prior to being gradually replaced within the community by English through a process of language shift that started in the 17th century. It is not known if the 1722 parish church of the established Church of Ireland preached locally in Irish, English or both, although the Church of Ireland had already translated the Bible and the Book of Common Prayer into Irish, which were used in Ireland and Scotland. Initially English was confined to trading with outsiders and amongst a small but growing number of newcomers during the 18th century, most islanders continued to speak Irish. Nevertheless knowledge and use of English increased and expanded on Rathlin until English eventually replaced Irish as the common community language probably some time in the middle of the 19th century. Irish persisted for a few decades as a family language amongst some islanders until they too switched to English. The last native Irish speakers on the island died in the 1950s and 1960s.

The local dialect shows many features typical for Scottish Gaelic and not found in other dialects of Ireland, e.g. forming plurals with -(e)an Irish pronunciation: [ənˠ], use of the interrogative có (standard Irish cé), it uses the object pronouns e, i for subjects of finite verbs, etc. It is nevertheless mutually intelligible with dialects of the Irish mainland in the Glens of Antrim. The features shared with Irish dialects are typically retentions that were lost in Scotland between the 17th and 20th century rather than innovations common with Irish, e.g. retention of eclipsis of p, t, c, f (but not b, d, g) after some words ending in a consonant (eg. nar bpeacaidh 'our sins', nar bpiúr 'our sister', seacht bpont 'seven pounds').

A ferry operated by Rathlin Island Ferry Ltd connects the main port of the island, Church Bay, with the mainland at Ballycastle, six nautical miles (eleven kilometres) away. Two ferries operate on the route – the fast foot-passenger-only catamaran ferry Rathlin Express and a purpose-built larger ferry, commissioned in May 2017, Spirit of Rathlin, which carries both foot passengers and a small number of vehicles, weather permitting. Rathlin Island Ferry Ltd won a six-year contract for the service in 2008 providing it as a subsidised "lifeline" service. There is an ongoing investigation on how the transfer was handled between the Environment Minister and the new owners.^{[needs update]}