The IRT Third Avenue Line, commonly known as the Third Avenue Elevated, Third Avenue El, or Bronx El, was an elevated railway in Manhattan and the Bronx, New York City. Originally operated by the New York Elevated Railway, an independent railway company, it was acquired by the Interborough Rapid Transit Company (IRT) and eventually became part of the New York City Subway system.

The first segment of the line, with service at most stations, opened from South Ferry to Grand Central Depot on August 26, 1878. Service was extended to Harlem in Manhattan on December 30. In 1881 this line already began the 24/7 service.Service in Manhattan was phased out in the early 1950s and closed completely on May 12, 1955. The remaining service in the Bronx was designated as part of the 8 route until it was discontinued on April 29, 1973.

The Third Avenue El was the last elevated line to operate in Manhattan, other than the 1 train on the IRT Broadway–Seventh Avenue Line (which has elevated sections between 122nd and 135th Streets and north of Dyckman Street), and was a frequent backdrop for movies. Service on the Second, Sixth and Ninth Avenue elevated lines were terminated in 1942, 1938, and 1940, respectively.