Nikki Bella Says Ronda Rousey's WWE Debut Was 'Slap In The Face'

Nikki Bella says she was PISSED WWE decided to introduce Ronda Rousey on the night of a historic all-women's event ... calling the move "a bit of a slap in the face" to the female wrestlers that came before her.

Of course, WWE hosted it's inaugural ladies-only Royal Rumble in Jan. 2018 ... with 30 wrestlers (including Nikki) duking it out for an hour straight. After the Rumble, "Rowdy" Ronda Rousey came out of the tunnel for the first time ever and pointed to a Wrestlemania sign hanging in the rafters.

The moment was supposed to tease Ronda's wrestling debut -- which went down at Mania in April '18 -- but Nikki felt the move essentially stole the thunder from the history-making Rumble.

While Bella says she has no beef with Ronda, she explains her frustrations in the Bella Twins' new book, "Incomparable," which came out this week.

"It was nothing against Ronda -- it is thrilling that she is at WWE -- but it was a bit of a slap in the face to all the historic women wrestlers who had come out for the match, the main event, only to have the moment upstaged by the Ronda announcement."

Nikki says she had planned to retire after the historic event due to a neck injury ... and it sucked to have her grand finale overshadowed by the UFC superstar.

"It just didn't need to happen like that," Nikki says in her book (by way of E!).

Nikki canned her plans and took a match after the Rumble -- and ultimately went on to retire later that year after losing to Rousey in a championship match for the Raw Women's belt at Evolution.