Belfast is situated on Ireland's eastern coast. The city is flanked to the northwest by a series of hills, including Cavehill, which is thought to be the inspiration for Jonathan Swift's novel, Gulliver's Travels. Belfast is located at the western end of Belfast Lough and at the mouth of the River Lagan making it an ideal location for the shipbuilding industry that once made it famous. When the Titanic was built in Belfast in 1912, Harland and Wolff had the largest shipyard in the world. Belfast is situated on Northern Ireland's eastern coast. A consequence of this northern latitude is that it both endures short winter days and enjoys long summer evenings. During the winter solstice, the shortest day of the year, local sunset is before 16:00 while sunrise is around 08:45. This is balanced by the summer solstice in June, when the sun sets after 22:00 and rises before 05:00.

Belfast is located at the eastern end of Belfast Lough and at the mouth of the River Lagan. In 1994, a weir was built across the river by the Laganside Corporation to raise the average water level so that it would cover the unseemly mud flats which gave Belfast its name (from the Irish: Béal Feirste meaning "The sandy ford at the river mouth"). The area of Belfast Local Government District is 42.3 square miles (110 km²).

The River Farset is also named after this silt deposit (from the Irish feirste meaning ‘sand spit’). Originally a more significant river than it is today, the Farset formed a dock on High Street until the mid 19th century. Bank Street in the city centre referred to the river bank and Bridge Street was named for the site of an early Farset bridge. However, superseded by the River Lagan as the more important river in the city, the Farset now languishes in obscurity, under High Street.

The city is flanked on the north and northwest by a series of hills, including Divis Mountain, Black Mountain and Cavehill thought to be the inspiration for Jonathan Swift's Gulliver's Travels. When Swift was living at Lilliput Cottage near the bottom of the Limestone Road in Belfast, he imagined that the Cavehill resembled the shape of a sleeping giant safeguarding the city. The shape of the giant's nose, known locally as Napoleon's Nose, is officially called McArt's Fort probably named after Art O'Neill, a sixteenth century chieftain who controlled the area at that time. The Castlereagh Hills overlook the city on the southeast.

Former poet and Catholic Bishop of Down and Connor, Dr William Philbin wrote this of Belfast: "Belfast is a city walled in by mountains, moated by sees, and undermined by deposits of history".