The Seven Legendary Riders (七伝説のライダー Nana Densetsu no Raidā?) are the first seven Kamen Riders: Takeshi Hongo (Kamen Rider 1), Hayato Ichimonji (Kamen Rider 2), Shiro Kazami (Kamen Rider V3), Joji Yuki (Riderman), Keisuke Jin (Kamen Rider X), Daisuke Yamamoto (Kamen Rider Amazon), and Shigeru Jo (Kamen Rider Stronger). The series these Riders originated from (Kamen Rider, Kamen Rider V3, Kamen Rider X, Kamen Rider Amazon, and Kamen Rider Stronger) were the first five series, a continuous run lasting from April 1971 to January 1976. An interesting point to notice is that all seven of these Showa-era Riders had their mechanic be Tobei Tachibana.
The seven Riders were first united during Kamen Rider Stronger. They also appeared as a group a few times in Kamen Rider (Skyrider), often shown operating separately from the Eighth Rider, Skyrider. It was only in Movie War Mega Max, however, a crossover movie between Kamen Rider Fourze and Kamen Rider OOO, that the group reappeared, separate from later Riders, and was referenced with a proper name by other characters.
The Seven Legendary Riders later appear again as a group in Super Hero Taisen where they take on Captain Marvelous (Gokai Red), although by the time of the final battle against Dai-Shocker and Dai-Zangyack, they're mixed with the various other Riders and Super Sentai heroes.