James Gallier (24 July 1798– 3 October 1866) was a prominent nineteenth-century Irish-born American architect, most famed for his buildings in New Orleans. Gallier Hall, which he designed and once served as New Orleans City Hall, is named after him.

He was born in Ravensdale, County Louth, Ireland in 1798 as James Gallagher, the son of Thaddeus Gallagher, a builder who also trained James in the profession.

Gallier was admitted to the "School of Fine Arts" in Dublin according to Supplement 1 of the Dictionary of American Biography, but if this refers to the Royal Dublin Society's School of Architectural Drawing, there is no record of a James Gallagher or James Gallier having been admitted at any other time. He worked in Manchester during 1816 before returning home to Ireland, where he attended Samuel Nielson's school in Dundalk, and with his younger brother John (b. 1800) engaged in building work at Mourne Park in 1818 and in Dundalk in 1821–22.

Gallier (then still known as Gallagher), returned to England in 1822 with his brother and worked for the next ten years in Huntingdon and London. In 1827, he designed the Godmanchester Chinese Bridge which crosses a mill stream of the River Great Ouse in 1827, and then worked on the redevelopment of the Grosvenor Estate in Mayfair, along with commissions for college buildings, prisons, and factories. During two of these years he apparently worked for famed Greek Revival architect William Wilkins (1778–1839). He went bankrupt, however, and decided to immigrate to the US in 1832.

Arriving in the US in New York, Gallier formed a brief partnership with his exact contemporary Minard Lafever (1798–1854), and published The American Builder's General Price Book and Estimator (1833). From Lafever undoubtedly Gallier met the brothers Charles (1811–1839) and James Dakin (1806–1852), who were then working for the prominent New York architects Ithiel Town and Alexander Jackson Davis. Town & Davis employed Gallier (then still called Gallagher) for four months in 1834 for the wage of $2.00 a day. Town and Davis, and James Dakin on his own in 1834, were in the midst of designing some of the most distinguished Greek Revival buildings in the United States at the time, including the Bank of Louisville in Kentucky (1834), as well as the First Presbyterian Church in Troy, New York (1835).

The American cities of the North were growing too crowded for many in the architectural profession, and in the 1810s, '20, and '30s many, including Benjamin Latrobe (1764–1820) and William Strickland (1788–1854), left Washington and Philadelphia, respectively, for southern destinations like Nashville and New Orleans, and Gallier and Charles Dakin followed suit in 1834, departing New York for New Orleans. According to one source, Gallier changed his last name at this time, probably to fit in better with the Francophone community there, which was still quite large and powerful; but according to Gallier's own autobiography, the name Gallier had been in the family since at least the seventeenth century. Despite jettisoning that particular piece of Irish heritage, Gallier still sought out his ethnic brethren as clients in the Irish community of Faubourg St. Mary, now known as the Central Business District.

In 1835 James Dakin soon joined his brother and Gallier in New Orleans and the three of them founded an architecture firm. Their firm quickly became one of the most important in the city and the Gulf region, even though much of their early work from the 1830s no longer survives. Amongst the earliest and most significant was the St. Charles Hotel (1835–37), which was one of the first large buildings constructed on Canal Street, which would develop into the city's main commercial artery. This impressive Greek Revival structure had a 6-column projecting Corinthian portico, marble front steps, and huge dome, second only in size in the US to the cupola of the United States Capitol in Washington, D.C. The ballroom in the basement was octagonal in plan, 70 feet (21 m) in diameter and 20 feet (6.1 m) high, encircled by Ionic columns; above this sat the lower saloon with a height of 18 feet (5.5 m), into which the marble staircase led. The saloon contained a marble statue of George Washington. From there, a grand spiral staircase continued up to the dome, with a gallery level stretching around it on each of the upper stories. The cupola itself was 46 feet (14 m) in diameter and sat on an octagonal drum; it was capped by a light well of Corinthian columns. At the top of the spiral stairs at the base of the dome was a large 11-foot (3.4 m) wide gallery, which afforded views of the entire city at a height of 185 ft (56 m). The hotel cost an enormous sum of $600,000 to build ($19.9 million in 2021), plus $100,000 for the land. It remained quite possibly the city's most prominent building and rivaled all others in the US until it burned on 18 January 1851; it was rebuilt in modified form by Thomas Sully.

During this period Gallier and the Dakins also designed The Second Christ Church Cathedral on St. Charles Avenue (1837), an Episcopalian Church and later a synagogue, but since demolished; Union Terrace" (1836–37) on Canal Street, of the State Arsenal (1839), and of the gracious row of thirteen houses on Julia Street known as the "Thirteen Buildings" or the Julia Street Row, where famed American architect Henry Hobson Richardson (1838–1886) would grow up. They also completed the Verandah Hotel (1837–38), and the Merchants' Exchange (1835–36) on Royal Street. In 1838 Dakin designed St. Patrick's Church, an ambitious effort in a rich Gothic style, supposedly modeled on York Minster. When difficulties occurred in its construction, Gallier was called in to revise the foundations and Dakin lost the contract in 1839. Ever afterward Gallier erroneously claimed it as one of his buildings. Charles also established a branch of the firm in Mobile, Alabama, and there the Dakins and Gallier completed Barton Academy in 1836 and Government Street Presbyterian Church (1836), now a National Historic Landmark. A large fire in Mobile in 1839 destroyed much of the firm's unfinished work there; that same year Charles apparently became depressed after the collapse of a row of warehouses he designed and left for Texas to start over, but died later in 1839 in St. Gabriel, Louisiana.