9:30 PM PT -- To quote the great Beyoncé: We gotta lay our cards down, down, down ... we got this one wrong.

Beyoncé is in Chicago, and getting ready to pop out for Kamala Harris on the final night of the Democratic National Convention ... TMZ has learned.

Multiple sources in the know tell us Queen Bey will be the big surprise performer as VP Harris officially accepts the Democratic party's nomination to run for president.

As you'd expect, Beyoncé's appearance is a huge deal, not only for Harris and the Party but in Chicago as well. We're told Chicago PD is on high alert as it's involved in security for Beyoncé at the United Center arena.

We don't know what she'll be performing, but the smart money is on "Freedom" ... as it's been the Harris-Walz campaign's anthem.

There have been rumors all week that either Bey or Taylor Swift would show up in Chicago before the DNC ended. In fact, there are dozens of delegates in the arena tonight dressed in "Cowboy Carter" styled outfits ... just in case Beyoncé were to come through.

Well, turns out that was more than a good bet on their part.

Now, we know what you're thinking -- what about Tay Tay? It would be a pretty amazing double-barreled attack if Kamala Harris were able to pull it off, but, at this point, it's a wait-and-see situation. As you know, Taylor wrapped up the European leg of her "Eras" tour Tuesday night in London ... so, she could be available.

Our DNC sources did confirm one other celeb appearance tonight -- NBA superstar Steph Curry recorded a message that will be played inside the arena, and will include his endorsement. Remember, Steph's Golden State Warriors coach Steve Kerr spoke earlier this week on night 1 of the DNC.