The Tanto is a traditional samurai dagger is often used as a secondary weapon to the Katana, and is great for close range combat and surprise attacks. As a secondary weapon to a Katana, they are known together as a "Daishō" (大小), literally meaning "Big-Small", with "Big" referring to a Katana and "Small" referring either to a Tantō or a Wakizashi (脇差).
The Tanto is a short dagger used by the Turtles. It is the smallest of the three traditional blades carried by a Samurai. The ones used by the Turtles are light, straight, and perfect for throwing. It appeared in episodes, such as New Friend, Old Enemy, TCRI, and Panic in the Sewers: In "New Friend, Old Enemy", Leo used these to cut Mikey free from his bonds when he was kidnapped by Chris Bradford and Xever. In "TCRI", Donnie used Leo's tanto to make a cut in the glass cell he and his brothers were trapped in when they were captured by the Kraang.
In "Slash and Destroy", Raph takes one out to use against Slash after seeing him beat Leo up, since his Sai were knocked out of reach earlier in the fight.
According to the Concept Art of Leonardo's Katana, "Niten Ryu", in the 2012 series, a Tanto blade is concealed within the hilt of the "Daito" (Long Sword), and a "Kogatana" (Mini Katana) is concealed within the Saya's (Scabbard's) binding wrappings, all as secondary, auxiliary weapons to his Katanas.