The Keikyu Main Line (京急本線, Keikyū-honsen) is a railway line in Japan, operated by the private railway operator Keikyu. The line connects the Tokyo wards of Minato, Shinagawa, Ōta, and the Kanagawa municipalities of Kawasaki, Yokohama and Yokosuka. The Keikyu Main Line began as a short 2 km (1.2 mi) line in 1895. By 1905 it was extended from Shinagawa Station in Tokyo to central Yokohama, becoming a major interurban line between the two cities.

Keikyu operates the following different types of service, including all-stations "Local" trains.

Abbreviations:

Morning Wing trains skip Yokohama Station, which is a major interchange railway station and is the fifth-busiest railway station in the world.

For connections and distances, see the route diagram.

All sections of the line were built as dual track. The Keihin Railway opened the Kawasaki to Omori section in 1901 as a 1,435 mm (4 ft 8+1⁄2 in) gauge line electrified at 600 V DC. In 1904, the line was regauged to 1,372 mm (4 ft 6 in) and extended to Shinagawa.

In 1930, the Shonan Electric Railway opened the Uraga to Koganecho section as a 1,435 mm (4 ft 8+1⁄2 in) gauge line electrified at 1,500 V DC. In 1931, the line from Yokohama was extended to connect at Koganecho. Freight services ceased in 1932, the line was regauged to 1,435 mm (4 ft 8+1⁄2 in) the following year, and in 1936, the voltage on the Shonan line was reduced to 600 V DC.

In 1941, the Shonan Electric Railway merged with the Keihin Railway, which merged with Tokyu the following year. The voltage on the entire line was raised to 1,500 V DC in 1945, and in 1948, the Keihin Electric Railway was created to operate the railway.