Belmullet (Irish: Béal an Mhuirthead, meaning 'mouth of the Mullet [Peninsula]', IPA:[ˌbʲeːlənˠˈwʊɾˠhəd̪ˠ]) is a coastal Gaeltacht town with a population of 1,019 on the Mullet Peninsula in the barony of Erris, County Mayo, Ireland. It is the commercial and cultural heart of the barony of Erris, which has a population of almost 10,000. According to the 2016 census 50% of people in the town were able to speak Irish while only 4% spoke it on a daily basis outside the education system.

Belmullet has two bays, Blacksod Bay and Broadhaven Bay, linked by Carter's Canal running through the town.

The origin of the name Belmullet is not clear. It may have come from Irish Béal Muileat or Béal an Mhuileat, which has been translated as "mouth of the isthmus". Bernard O'Hara in Mayo: Aspects of its Heritage suggests that "A change from 'L' to 'R', which is quite common in Irish, may have given Béal an Mhuireat which in turn became Béal an Mhuirhead". It has also been suggested that the latter half of the name may refer to the fish or the star shape used in heraldry.

According to Richard Pococke, in about 1715, Sir Arthur Shaen "began building a little town" where Belmullet now stands.^{[citation needed]} During the reign of Queen Elizabeth I, an admiral chased pirates into Broadhaven Bay, hauled his boats across the isthmus, and caught up with them near the Iniskea Islands. To drain the area and form a passageway from Blacksod Bay into Broadhaven Bay, Shaen had a canal excavated, known thereafter as Shaen's Cut, large enough for small boats to pass through from one bay to the other. However, little development of the town occurred, and by 1752 the canal was choked up and impassable. Belmullet was the scene of Monster meetings of the Land League at the end of the 19th century. In the early 19th century Belmullet consisted of little more than a few thatched buildings.^{[citation needed]}

In 1820, the first post office in Erris opened in the new town of Belmullet. In 1822, a coastguard station was built. William Henry Carter had inherited huge tracts of Shaen's land in Erris when he married Shaen's daughter and began to put plans in place to develop the town. Carter tasked engineer and Castlebar native Patrick Knight to design and plan the new town between Broadhaven and Blacksod bays. Knight published his town plan in his 1836 treatise "Erris in the Irish Highlands," where he described Belmullet as a 'mirage in the desert', positioned between two sheltered bays which provided potentially safe anchorage for Atlantic trade. A new road was built which connected Belmullet with Castlebar, which was completed in 1824. In 1825, Carter built a pier large enough to accommodate vessels of 100 tons. Carter's stated objective was to "create a home market for produce that did not previously exist nearer than thirty miles by land," and his aim was to thrust the older village of An Geata Mór (Binghamstown), a village founded by the powerful Bingham family on the Mullet peninsula into a secondary position. By 1826, the Erris Hotel was opened.

In 1829, Alexander Nimmo, an engineer on the Erris roads, wrote the following: "at Belmullet, the advance is quite surprising; the place only commenced four years ago; it now consists of about seventy respectable houses etc... five ships were loaded with grain and kept; iron hoops and coal were imported; spirits, beer and wine. British manufacturers and tea and sugar were sold; the produce of the fisheries were admitted to a market." In 1836, Patrick Knight documented in his book "Erris in the Irish Highlands" that during the construction of the Belmullet-Bangor road, workers "unnecessarily destroyed the fine dolmen in the glen rather than divert the road a few feet to one side".

By 1831, the population of Belmullet was 585. A Roman Catholic chapel was built in 1832 at the cost of £300. There was a daily postal service between Ballina and Belmullet. In 1833, a courthouse was built also costing £300 which held weekly court sessions, demonstrating that the town was rapidly growing. In the 1830s, a visitor described it as "the youngest town in Ireland and like all young things it is comparatively fresh and fair. The town itself contains a few thatched cabins but consists of small streets of moderately sized slated houses branching from a little square, or market place; the shops looked to be well furnished with not only necessaries but articles conducive to comfort and convenience. Buildings are going on and speculation is progressing." He also commented that the approach to the town was "spoiled by deformed, wretched bog huts." Two new roads were built – one to the east went to Ballycastle and one to the south to Newport. The export of meal from the area to England started. The local Protestant Church was built in 1843. In 1845, work began to re-open the canal which had been constructed by Arthur Shaen. Because of the Irish Famine, the canal was not completed until 1851. During the Relief work for the Distress (in the middle of the Great Famine) in 1846 and 1847, the footpaths were formed and flagged.

In the latter half of the 19th century rural unrest was a common occurrence across Mayo. In November 1881, over 100 police officers needed in order to protect two process servers who were serving writs in the town. The police were confronted by an angry crowd throwing rocks and sticks. The police were obliged to charge the crowd and fire buckshot resulting in a large number of injuries. The police made 20 arrests.