The Art Gallery of Ontario (AGO; French: Musée des beaux-arts de l'Ontario) is an art museum in Toronto, Ontario, Canada. Located on Dundas Street West in the Grange Park neighbourhood of downtown Toronto, the museum complex takes up 45,000 square metres (480,000 sq ft) of physical space, making it one of the largest art museums in North America and the second-largest art museum in Toronto, after the Royal Ontario Museum. In addition to exhibition spaces, the museum also houses an artist-in-residence office and studio, dining facilities, event spaces, gift shop, library and archives, theatre and lecture hall, research centre, and a workshop.

Established in 1900 as the Art Museum of Toronto and formally incorporated in 1903, the museum was renamed the Art Gallery of Toronto in 1919, before adopting its present name, the Art Gallery of Ontario, in 1966. The museum acquired the Grange in 1911 and later undertook several expansions to the north and west of the structure. The first series of expansions occurred in 1918, 1924, and 1935, designed by Darling and Pearson. Since 1974, the gallery has undergone four major expansions and renovations. These expansions occurred in 1974 and 1977 by John C. Parkin, and 1993 by Barton Myers and KPMB Architects. From 2004 to 2008, the museum underwent another expansion by Frank Gehry. The museum complex saw further renovations in the 2010s by KPMB and Hariri Pontarini Architects.

The museum's permanent collection includes over 120,000 works spanning the first century to the present day. The museum collection includes a number works from Canadian, First Nations, Inuit, African, European, and Oceanic artists. In addition to exhibits for its collection, the museum has organized and hosted many travelling art exhibitions.

The museum was founded in 1900 as the Art Museum of Toronto by a group of private citizens and members of the Toronto Society of Arts. The institution's founders included George A. Cox, Lady Eaton, Sir Joseph W. Flavelle, J. W. L. Forster, E. F. B. Johnston, Sir William Mackenzie, Hart A. Massey, Professor James Mavor, F. Nicholls, Sir Edmund Osler, Sir Henry M. Pellatt, George Agnew Reid, Byron Edmund Walker, Mrs. H. D. Warren, E.R. Wood, and Frank P. Wood.

The museum's incorporation was confirmed by the Government of Ontario three years later by legislation, in An Act respecting the Art Museum of Toronto in 1903. The legislation provided the museum with expropriation powers in order to acquire land for the museum. Before the museum moved into a permanent location, it held exhibitions in rented spaces belonging to the Toronto Public Library near the intersection of Brunswick Avenue and College Street.

The museum acquired the property it presently occupies shortly after the death of Harriet Boulton Smith in 1909, when she bequeathed her historic 1817 Georgian manor, The Grange, to the gallery upon her death. However, exhibitions continued to be held in the rented spaces at the Toronto Public Library branch until June 1913, when The Grange was formally opened as the art museum. In 1911, ownership of The Grange, and the surrounding property was formally transferred to the museum. Shortly afterwards, the museum signed an agreement with the municipal government of Toronto to maintain the grounds south of The Grange as a municipal park.

In 1916, the museum drafted plans to construct a small portion of a new gallery building designed by Darling and Pearson in the Beaux-Arts style. Excavation of the new facility began in 1916. The first galleries adjacent to The Grange were opened in 1918. In the next year, the museum was renamed the Art Gallery of Toronto, in an effort to avoid confusion with the Royal Ontario Museum, itself also an art museum. In 1920, the museum also allowed the Ontario College of Art to construct a building on the grounds. The museum was expanded again in 1924, with the opening of the museum's sculpture court, its two adjacent galleries, and its main entrance on Dundas Street. The museum was expanded again in 1935 with the construction of two additional galleries. Portions of the 1935 expansions were financed by department store chain Eaton's.

In 1965, the museum saw its collection of European and Canadian artworks expand, with the acquisition of 340 works from the Canadian National Exhibition. During the mid-1960s, the director of the museum, William J. Withrow, pushed to have the museum designated as a provincial museum, in an effort to gain further provincial funding for the institution. In 1966, the museum changed its name to the Art Gallery of Ontario, in order to reflect its new mandate to serve as the provincial art museum.