Nikki Bella just gave Indianapolis a huge surprise ... as she's officially made her return to WWE at the "Royal Rumble."
One-half of the Bella Twins made her appearance minutes ago ... entering the women's match at the number 30 spot -- with the crowd going nuts when her theme song hit the loudspeakers.
Bella was decked in an all-red two-piece outfit and ball cap ... with her signature "Fearless" logo etched across her chest.
She proved there's no ring rust .