Reese Witherspoon, Demi Moore and Nicole Richie were all at the party which was a joint bash for Kate and friend Jen Meyer, and was held in the Los Angeles bar No Name Bar.
Fashion editor Derek Blasberg posted several snaps on his Instagram account, including one that showed Kate and Jen, who is married to Tobey Maguire, blowing out their birthday candles.
Kate Hudson and Jen Meyer blow out birthday candles
"Happy birthday, Jen and Kate!," he captioned the shot.
Earlier in the night he also posted a picture of Reese, Jen and actress Jessica Capshaw with friends, writing next to it: "Reese, Jessica, Kelly, Jen and Emily: Blondes. Having more fun. Happy birthday!"
Kate wore a metallic jumpsuit with a neckline that plunged to her navel and reportedly kept warm with a velvet blazer, while Reese stunned in a bright pink dress.
Kate's husband Matt Bellamy was also there and one eyewitness told E! News that the pair looked radiant with love.
"They held hands and kissed throughout the night," the source said, before adding: "It was a really fun night, everybody had a blast."
"Reese, Jessica, Kelly, Jen and Emily: Blondes. Having more fun. Happy birthday!"
The 33-year-old Oscar-nominated Almost Famous actress has been engaged to Matt since 2011, and the pair live together with Kate's child from her first marriage and their two-year-old son.
The gorgeous Kate was recently a shoulder to cry on for Lea Michele, opening up her home to her Glee co-star after the death of Cory Monteith.
"I called her and said, 'I don't know where I'm going to go because my house is swarmed (with reporters)," the 27-year-old told the December 2013 issue of Elle. "She was like, 'Oh, you're going to stay at my house.' Like it was nothing."
Mum-of-two Kate became close to Lea when she joined the cast of Glee in 2012 as dance teacher Cassandra July.