Lordy, lordy, Reese Witherspoon is 40!

The Oscar-winning actress let her hair down and hopped on stage to sing "Sweet Home Alabama" -- accompanied by Keith Urban on guitar -- at her star-studded 40th birthday party on March 19.

Vanity Fair writer Derek Blasberg posted a clip of Reese clad in a sparkling black mini-dress as she belted out the song on stage with the country star playing the classic tune behind her (Reese, you'll recall, starred in the 2002 rom-com "Sweet Home Alabama" with Josh Lucas and Patrick Dempsey).

"You can take the girl out the South, but you can't take Sweet Home Alabama out the girl! HAPPY BIRTHDAY@reesewitherspoon️," Derek captioned the Instagram video.

Reese's guests also included her mom, Kate Hudson, Tobey Maguire's wife Jennifer Meyer, Laura Dern and Taylor Swift, whom E! reports sang her hit "Shake It Off" at one point in the evening.

Kate had a blast at the party but found herself photo-challenged. "When you take a s--- load of pictures and you still don't have it perfect so you go for the ole collage post.... Happy Birthday gorgeous girl@reesewitherspoon !!! Love you πŸ‘ΈπŸ’ƒ," Kate captioned an Instagram post of herself with the birthday girl and some of the revelers before adding a slew of hastags. "#TequillaFirst πŸ‘ #PostingLater πŸ˜³πŸ™ˆ#SaturdayNight#ImSoExcitedToGetIntoBedIDontEvenKnowWhatToDoWithMyself #NotWashingMyFace#GoodNight βœ¨πŸ’‹"

Reese also shared some b-day photos on her own Instagram page, including a selfie with the caption, "Happy Saturday y'all!! πŸŽ‰#BirthdayWeekend."

Happy Saturday y'all!! πŸŽ‰#BirthdayWeekend

A post shared by Reese Witherspoon (@reesewitherspoon) on

The mother of three officially turns 40 on March 22.