The London Underground 1973 Stock is a type of rolling stock used on the Piccadilly line of the London Underground. It was introduced into service in 1975 with the extension of the line to Hatton Cross, followed by a further extension to Heathrow Central in 1977. A total of 86 six-car trains were built.

The trains were built by Metro-Cammell between 1974 and 1977 and were refurbished by Bombardier Transportation between 1996 and 2001. They are some of the oldest trains running on the Underground, and in Britain as a whole, second only to the 1972 Stock running on the Bakerloo line.

The initial order included two test units equipped with solid state traction equipment and electronic control systems. These were double-ended units 892-692-893 (delivered 1977) and 894-694-895 (delivered 1979) and were known collectively as the ETT (Experimental Tube Train). The first unit was equipped by Westinghouse, the second by GEC. In order to provide additional units for the opening of the Heathrow loop, these units were converted to standard at Acton Works, entering service between 1986 and 1987.

In June 2018, TfL announced 94 nine-car 2024 stock trains to replace the 1973 Stock. As of early 2021, these are expected to enter service from 2025.