Niagara is an oil painting produced in 1857 by the American artist Frederic Edwin Church. Niagara, which portrays the Horseshoe Falls portion of Niagara Falls, was Church's most important work at the time and confirmed his reputation as the premier American landscape painter of the era. In his history of Niagara Falls, Pierre Berton writes, "Of the hundreds of paintings made of Niagara, before Church and after him, this is by common consent the greatest."

The Falls were commonly painted, being such an attraction to landscape artists that, writes John Howat, they were "the most popular, the most often treated, and the tritest single item of subject matter to appear in eighteenth- and nineteenth-century European and American landscape painting". Moreover, the public was captivated by the natural wonder of the Falls, considered a landmark of the North American landscape and a major tourist destination. It was the "Honeymoon Capital of the World", and prints of Niagara were given as wedding gifts. In the 1850s, Niagara was the subject of millions of stereographs, and its image could be found on wallpaper, china, and lampshades, among other consumer items. In 1853, a 1600-foot moving panorama of Niagara Falls was exhibited in New York City.

For Americans, the Falls symbolized the grandeur and expansionism of the United States. David C. Huntington, whose writing on Church in the 1960s revived interest in the painter, explained how Americans, in an era of spiritual optimism and manifest destiny, would have perceived such a vivid painting of Niagara, with all that it symbolized:

Church presented his fellow-men with the "soul" and "spirit" of Niagara, this "most suggestive" of nature's spectacles: this archetype of the universe. Niagara is the substance of a great American metaphor; indeed, for its original viewers, a certain something more than a metaphor.... Nature and its Bible, the Science of Design, would unfold the transcendent truth of the universe to the New Chosen People in the New World. Nature in the Era of Manifest Destiny was prophecy, and Church as the "interpreter" of Niagara was therefore painting as American prophet. Niagara is the American's mythical Deluge which washes away the memory of an Old World so that man may live at home in a New World. The painting is an icon of psychic natural purgation and rebirth. Poetically a New World emerges as the waters of a flood subside. The rainbow, sign of the "God of Nature's" covenant with man, transfixes the beholder.... Niagara is a revelation of the cosmos to each and every man. Before this greatest of American landscapes the self-reliant, democratic American becomes his own prophet: he stands and sees as a New Noah. Thus through the work of art did Mr. Church help his fellow-men to discover themselves in their New World.

Church studied Niagara Falls extensively leading up to 1857, making dozens of pencil and oil studies. In 1856 Church visited the Falls for probably the fifth time. His teacher Thomas Cole had also visited the Falls. A writer in Art and Artists in Connecticut (1879) reported that the painting itself took about six weeks and that Church used two similar canvases simultaneously: a "draft" upon which he tested a painting idea, and the final canvas, to which he transferred the results he found satisfactory.

Church's painting is of Horseshoe Falls, the largest and most iconic of Niagara's three waterfalls. With a width of 2.3 metres (7 ft 7 in), it is more than twice as wide as it is high. The canvas's unusual proportions allowed him to paint a panoramic view from the Canadian side of the falls; the composition leads the eye laterally. The vantage point was dramatic and unique, leaving behind the "canonical banality" of many other paintings before it, the merely picturesque, and immersing the viewer directly in the scene, as if airborne or even in the water. The lack of a repoussoir aids this effect. Before the advent of modern photography, which made such images common, this was a revelation.

Niagara is highly naturalistic, more so than Church's previous The Andes of Ecuador (1855), and shows the influence of John Ruskin's aesthetics on Church. Church brings the viewer to the lip of the falls, highlighting the impressive drop by painting in streams of water and cloudy mists. The only foreground object is a floating tree trunk, which might be confused for a branch but for its roots, providing a sense of scale. The white foam near the trunk has some build-up of paint on a canvas that is otherwise smooth. The foam might suggest that the tree is caught on an unseen rock; there is ambiguity in whether this location is a small respite of stability or highlights the imminent danger of reaching the fall's edge.

Church's extensive study of the falls allowed him to capture the effect of mist and turbulent water with unprecedented realism. The light creates a partial rainbow beyond the precipice, whose arc is strong where the mist is thick, and absent elsewhere, a highly realistic rendering and a technical achievement. Distant on the horizon are a number of buildings, including Terrapin Tower, on the platform of which stands a tiny person.