Diaz, who won Season 5 of The Ultimate Fighter, returned to the win column after most recently being defeated by Rory MacDonald at welterweight.
The Cesar Gracie-trained fighter is back at his natural fighting weight and he put on the best performance of his career.
Gomi had no answers for Diaz on the feet and a flawless display of jiu-jitsu proved to be too much for the former PRIDE champion.
Diaz' six-inch reach advantage kept Gomi on the outside where he was picked apart. Once the action hit the ground, Diaz quickly transitioned from a triangle attempt to a fight-ending armbar.
Gomi has only seen victory once in four attempts during his stint in the UFC and "The Fireball Kid" couldn't return to the form that saw him knockout Tyson Griffin.
With that being said, Diaz now begins his run towards the top of a stacked lightweight division.