Who invented the Rasengan?

Namikaze Minato.

The Rasengan is a ninjutsu created by Namikaze Minato over three years using the inspiration he obtained from the Tailed Beast Jade. The Rasengan can condense the chakra in the body into the hand, and then condense it into a chakra ball in a strange way to attack. Once hit, there will be a brief period of confusion.

The sound generated by Namikaze Minato and Naruto's chakra can use the most powerful Rasengan, the Taichi Rasengan.