Queen Bey made a surprise, unannounced appearance at Taylor Swift's now legendary premiere for her "Eras" tour concert film at The Grove in L.A. Wednesday night.

Taylor looked stunning in a $12,000 blue dress created by none other than Oscar de la Renta. And Beyoncé was the epitome of cool in her all-black bodysuit, chrome bustier and dark sunglasses.

Both superstars gave thousands of "Swifty" fans an even bigger treat -- posing together for photos on the red carpet in front of a wall of throwback images of Taylor and a huge ad for her movie.

Everyone then piled into the theater to watch the movie -- of course, Beyoncé and Taylor had a front-row seat and they proved it by posting an Instagram video of themselves sitting next to each other. Beyoncé playfully tossed popcorn on the ground as Taylor stuffed a kernel into her mouth.

In the caption, Taylor partly wrote, "I'm so glad I'll never know what my life would've been like without @beyone's influence. The way she's taught me and every artist out here to break rules and defy industry norms."

TMZ first reported ... Taylor was planning to pop up at the premiere after questions swirled about whether she would attend. The music icon took selfies with 2,200 of her excited "Swifties," who were lucky enough to be handpicked to watch the movie with her.

What's more .. the security was totally insane at The Grove, which shut down for the first time in 2 decades for the event.

Hundreds of guards were posted outside the shopping center to keep an eye on things. Barricades were also set up all around the perimeter.

And everything went according to plan.

Don't forget ... Taylor moved the release date for the film from Friday to today. Get your tickets now, Swifties!!!

Do you think Taylor Swift will show up at Beyoncé's movie premiere? Vote below.