Russell Wilson -- Crashes Ciara's Girls Night Out ... And She's Smitten

Girls night out isn't for BF's ... unless you're Russell Wilson ... 'cause TMZ Sports has learned the Seahawks QB surprised Ciara at a fancy steakhouse ... and she could not have been happier.

Ciara and two girlfriends were having dinner Saturday night at The Palm -- one of D.C.'s best restaurants -- drinking wine and eating crab ... when Wilson unexpectedly arrived during dinner.

The restaurant G.M. Mike Melore tells us ... "Ciara was shocked to see him. She said she didn't know he was coming, but she was super happy he was there."

And, when you're rich and famous ... you can stay as late as you want.

"We normally close around 10:30, but we stayed open late for them, until 1:30 AM," Melore says.

"They were extremely nice and humble. They took pictures with every staff member that asked before they left."