Sasaki Kojirō (佐々木 小次郎 died April 14, 1612) was a prominent swordsman; he is most famous for his death in battle with Miyamoto Musashi.

He carried a straight-edged nodachi (long two-handed sword) which he dubbed the 'Drying Pole'. He went by the fighting name of Ganryu (巌流). His trademark strike was respected and feared throughout feudal Japan, the dreaded 'Swallow's tail strike' (燕返し) mimicking the motion of a swallow's tail during flight, a cut reputedly so quick, so deadly and so accurate it could strike down a bird in mid flight.

Sasaki Kojiro was a long-time rival of Miyamoto Musashi, and is considered the most challenging opponent Musashi ever faced. According to the legend (it is disputed how much of the story is based on fact), he came near to victory several times before, blinded by the sunset behind his opponent, he was struck down with a bokken (wooden sword) which Musashi had fashioned out of the spare oar of the boat in which he arrived at the duel.

The debate still rages today as to whether or not Musashi cheated in order to win that fateful duel or merely used the environment to his advantage.

The male half of Team Rocket in the Pokémon anime (Kojirō) was named after him (in English, he is known as James).

In the game Brave Fencer Musashi, the title character's sworn rival was named after Kojiro. And in Musashi: Samurai Legend, the main villian was named Gandrake, named after 'Ganryu'.