On February 8, 2003, the Fox Network revived the Teenage Mutant Ninja Turtles franchise with the help of 4Kids Entertainment as a Saturday morning cartoon in the "FoxBox" programming block, which was subsequently renamed "4Kids TV". The series was produced by Mirage Studios, and Mirage owns one-third of the rights to the series. Mirage's significant stake in creative control resulted in a cartoon that hews more closely to the original comics, creating a darker and edgier feel than the 1987 cartoon, but still remaining lighthearted enough to be considered appropriate for children.
The series still includes some of the old villains, like the Shredder and Baxter Stockman, but added new villains as well as new friends. Leatherhead helps Donatello in a vital story arc and Karai takes over after her father the Utrom Shredder is exiled far away. April and Casey are also in this series and April has an antique shop like in the first film and the Mirage comics.
- Leonardo (blue) - Leonardo is courageous, decisive, and a devoted student of martial arts. As a strict adherent to Bushido, he has a very strong sense of honor and justice. He wears a blue mask and wields a pair of ninjaken. He is named after Leonardo Da Vinci.
- Raphael (red) - The team's anti-social bad boy, Raphael has an aggressive nature and seldom hesitates to throw the first punch. He is an intense fighter. His personality can be alternately fierce and sarcastic. Still, he is intensely loyal to his brothers and sensei. He is good friends with Casey Jones, after having met him one night on patrol and challenging him to a fight. The two have since frequently patrolled together. Raphael wears a red mask and wields a pair of sai. He is named after Raphael Santi.
- Michelangelo (orange) - The easy-going and free-spirited Mikey provides much of the comic relief. While he loves to relax, this Turtle also has an adventurous and creative side. He wears an orange mask and wields a pair of nunchaku. He is named after Michelangelo Buonarroti. His name was originally misspelled "Michaelangelo" by Peter Laird and Kevin Eastman -- at the time they were using a typewriter and had no way to perform spell-check.
- Donatello (purple) - The brilliant scientist, inventor, engineer, and technological genius. He is perhaps the least violent Turtle, preferring to use his intellect to solve conflicts. He wears a purple mask and wields the bō. He is named after the sculptor Donatello.
- Master Splinter - The Turtles' sensei and adoptive father, Splinter is a mutant rat who learned the ways of ninjutsu from his own master, Hamato Yoshi. In the 1987 series, Splinter was Hamato Yoshi himself mutated into a rat.
- April O'Neil - A former lab assistant to the mad scientist Baxter Stockman, April is the plucky human companion of the Turtles. She embarks on many of the Turtles' adventures and aids them by doing the work that the Turtles themselves cannot do in public. In the 1987 series, April was a television news reporter.
- Shredder - A villainous ninjutsu master called Oroku Saki. In every incarnation of the TMNT franchise, he has been the archenemy of Splinter and the Turtles. He is also the leader of the Foot Clan.
- Casey Jones - A vigilante who has become one of the Turtles' closest allies, Casey fights crime with an assortment of sporting goods (baseball bats, golf clubs, hockey sticks, cricket bat etc.) while wearing a hockey mask to protect his identity.