504 King is an east–west Toronto streetcar route in Ontario, Canada. It serves King Street in Downtown Toronto as well as Broadview Avenue on the east end and Roncesvalles Avenue on the west end of the line. The route consists of two overlapping branches: 504A between Line 2 Bloor–Danforth's Dundas West station and Distillery Loop, and 504B between Broadview station – also on Line 2 – and Dufferin Gate Loop. The two branches overlap on King Street between Dufferin and Sumach streets, both passing St. Andrew station and King station on subway Line 1 Yonge–University.

The Toronto streetcar system is a network of eleven streetcar routes in Toronto, Ontario, Canada, operated by the Toronto Transit Commission (TTC). It is the third busiest light-rail system in North America. The network is concentrated primarily in Downtown Toronto and in proximity to the city's waterfront. Much of the streetcar route network dates from the 19th century. Three streetcar routes operate in their own right-of-way, one in a partial right-of-way, and six operate on street trackage shared with vehicular traffic with streetcars stopping on demand at frequent stops like buses. Since 2019, the network has used low-floor streetcars, making it fully accessible.