Taylor Swift Performs at Reese Witherspoon's Birthday Party!

Reese Wotherspoon stops to pick up some magazines on Saturday afternoon (March 16) in Brentwood, Calif.

Later that night, the 39-year-old actress celebrated her birthday with a huge bash at Warwick in Hollywood. (Reese turns 40 on March 22.)

According to People, Taylor Swift performed at the event. Check out a video that Kate Hudson Snapchatted below!

Also in attendance were Jennifer Aniston and Justin Theroux, Courteney Cox, Matthew McConaughey, Robert Downey Jr., Nicole Kidman and Keith Urban, Tobey Maguire, Jason Bateman, and more.

“When you take a sh-t 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 �’��’� #TequillaFirst �’� #PostingLater 😳🙈 #SaturdayNight #ImSoExcitedToGetIntoBed IDontEvenKnowWhatToDoWithMyself #NotWashingMyFace #GoodNight ✨�’�”,” Kate captioned an Instagram collage.

15+ pictures inside of Reese Witherspoon out and about before her party…