Eglinton Avenue is a major east–west arterial thoroughfare in Toronto and Mississauga in Ontario, Canada. In the west, the street begins at Highway 407 (but does not interchange with the tollway) at the western limits of Mississauga, as a continuation of Lower Baseline in Milton and runs nearly 60 kilometres (37 miles) across the midsection of both cities before ending at Kingston Road. Within Toronto, Eglinton Avenue is the only street that crossed all six municipalities of the former Metropolitan Toronto prior to its amalgamation: Etobicoke, York, (Old) Toronto, East York, North York, and Scarborough. The section between the Etobicoke Creek and Renforth Drive forms part of the city limits of Toronto and Mississauga.

It was historically known as Richview Sideroad in Etobicoke and Lower Baseline in Mississauga. It was also briefly designated Highway 5A (and later Highway 109) in Scarborough.

The Toronto section was surveyed in the 19th century as the Fourth Concession Road (with the first being Queen Street).

There are two sources for the naming of Eglinton Avenue. Henry Scadding in an early history of the city wrote that it originated from Eglinton Castle in Scotland, itself named for the Earls of Eglinton. Several early settlers, impressed by the Eglinton Tournament of 1839 hosted by the 13th Earl, named the hamlet developing in the area as the Village of Eglinton after the Earl. More likely is the humbler story that it was named by the tavern keeper John Montgomery who settled in the area in 1830 and named the village after the Earl of Eglinton of the Montgomerie family, to whom he believed he had a family connection, despite the slight difference in spelling.

The wagon trail connecting to Yonge Street between the third and fifth concessions (St. Clair and Lawrence Avenues respectively) soon adopted the name of the village and was gradually improved over the years near Yonge Street. In 1890, the area was incorporated as North Toronto, and in 1912, it was annexed to Toronto itself. In 1953, Metropolitan Toronto (commonly known as Metro) was formed. Seeking to build new connections to the rapidly developing suburbs, Metro widened and interconnected Eglinton Avenue to its current form through the decade.

The eastern segment through Scarborough was known as Highway 5A between 1937 and 1953; this number also appeared on St. Clair Avenue West until 1952 when the Toronto Bypass, (the precursor to Highway 401), opened between Weston and Highway 11 (Yonge Street). The two pieces of "Highway 5A" were never connected. In 1953, what remained was renumbered as Highway 109; a year later, the road was removed from the provincial highway system. Because of its time as a provincial highway, the road through Scarborough was widened considerably. A right of way was also acquired to bridge the gap in Eglinton Avenue. Until the mid-1950s, Eglinton Avenue did not cross either of the valleys of the Don River. The road ended at Brentcliffe Road (unassigned path beyond Laird Drive towards Brentcliffe Road and the dump was once site of hangar for Leaside Aerodrome) and resumed at Victoria Park Avenue (then known as Dawes Road). This break resulted in a bypassed eastern stub at Bermondsey Road signed as Old Eglinton Avenue. The Department of Highways relinquished control of Highway 109 to the newly formed Metro government. Metro built the new section of Eglinton Avenue, first between Dawes Road and Don Mills Road in 1955, and later between Don Mills Road and Leaside in 1956.

In the west, the street ended at the Humber River until 1970. On the opposite side in Etobicoke, the Richview Sideroad followed the same alignment as far as the Metro Toronto–Peel boundary. That year, the river was bridged to connect the Richview Sideroad segment as an extension of Eglinton Avenue. At about the same time, when the Highways 401 and 427 interchange was reconstructed, the street was connected to Lower Base Line, extending the street even further west into Mississauga. West of Mississauga, in the Regional Municipality of Halton, it is a rural road and is still named Lower Base Line (the corresponding Upper Base Line being present-day Steeles Avenue).

The structure over the Richmond Hill line and East Don River is known as the Harvey C. Rose Bridge, and honours the chief engineer of the Toronto and York Roads Commission, later the Metropolitan Toronto Commission of Roads.