Rousey had hoped for a glorious return to UFC having taken a year out after her knockout defeat to Holly Holm in November last year.
But the former bantamweight champion was outclassed by an in form Leoa, who landed a series of devastating blows within the first minute.
From the off the Brazillian wrecking machine dominated, pounding Rousey’s face in the UFC 207 main event in LA.
Meanwhile, Rousey struggled to land a single blow, allowing Nunes to land 23 of the 35 punches and kicks she launched.
With Rousey overwhelmed, referee Herb Dean had no choice but to call time on the bout.
She said: “I knew I was going to beat the s*** out of Ronda Rousey. Now I proved to everyone that I’m the best in the world.
“This is my moment. She has had her time, she did a lot for the sport. But right now I am the champion.”
Nunes retained her women’s bantamweight championship after taking it from Miesha Tate at UFC 200.
Ronda Rousey focuses on her opponent Holly Holm
“I knew I was going to beat the s*** out of Ronda Rousey.”
The embarrassing loss will spark rumours Rousey may consider retiring from the sport.
Nunes added: “Now she’s going to retire and she’s going to go do movies, she already has a lot of money. In this division, we have a lot of talent.