Kobayakawa Hideaki (小早川 秀秋, 1577-1 December 1602) was a daimyo of the Sengoku period.
Fifth son of Kinoshita Iesada and nephew of Hideyoshi Toyotomi, he was adopted by the latter and took the name of Hashiba Hidetoshi (羽柴 秀俊), then his uncle had him adopted again by Kobayakawa Takakage to take over the inheritance of that clan.
He participated in the Korean invasion campaign, following which, after various plots between Ishida Mitsunari and Tokugawa Ieyasu, he was deprived by Hideyoshi of a large part of his estate, the latter being reduced from 336,000 to 120,000 koku and relocated from Kyūshū to Echizen.
Before he died, Hideyoshi reinstated Hideaki to his former domains. Ieyasu Tokugawa, on the other hand, arranged for Hideaki's disgrace to be blamed on Ishida Mistunari and his reinstatement on his own account.
At Hideyoshi's death, Hideaki found himself in favorable conditions to change sides for Ieyasu. He seems to hesitate for a long time before the battle, is close to join officially the armies of the East, before Ishida Mistunari manages to keep it on his side.
However, he sent a letter to Ieyasu Tokugawa before the battle of Sekigahara, to assure him that he would betray his allies.
During the battle, however, he still seems hesitant, ignoring Mitsunari's orders to charge the flank of Ieyasu's troops, and then Ieyasu's pleas for help, but ends up, under a cloud of warning arrows from Tokugawa, effectively changing sides, charging Ōtani Yoshitsugu's troops.
His betrayal, subsequently generating the reversal of several commanders of the western armies, is considered a key moment in the battle and the installation of the Tokugawa shogunate.
However, he received only a small improvement in his income of 50,000 koku. He is supposed to have become insane before his death in 1602, without any heir, making him the last daimyo of the Kobayakawa clan.