Speedster Island is a location that appears in the Sonic Underground television series. It is a small island located in the South Mobian Sea and the native home of Mantu, his father, and the rest of their people.
For ages, the inhabitants of Speedster Island have lived on the island in peace, with fishing seemingly being a big part of their customs. Eventually however, Sleet, Dingo, and Bartleby came to negotiate with the Chief about selling the island so the three could build a resort. Mantu, upset that his father would sell the island, asked the island's inert Guardians for aid. Unbeknownst to anyone, Sleet planned on roboticizing all inhabitants on the island.
When the Sonic Underground arrived on the island upon instructions from their mother, Mantu mistook them for the Guardians coming to help them, and asked them to save the island from coming under Dr. Robotnik's rule, which the heroes accepted. When trying to sneak into Bartleby’s residence, Sonic and Sonia ran to the Guardians when Swatbots, Sleet and Dingo spotted them. There, Manic had found three musical notes carved into the Guardians. Playing the three notes that Manic had uncovered, the Sonic Underground brought the Guardians to life, who destroyed the Swatbots and forced Sleet, Dingo, and Bartleby to run away. Assuring Mantu that they could now call upon the Guardians to protect them by playing the notes, the Sonic Underground left Speedster Island.