Bellamy, who has a son, Bingham, with the "Reluctant Fundamentalist" star, invited celebrity friends like Cameron Diaz and Judd Apatow to the gathering.
"Kate was totally surprised," a source told Us magazine. "The party started outside, but it was freezing so people moved inside and danced."
Hudson, who turned 34 on April 19, became engaged to Bellamy in 2011.
"It was very sweet. It was very romantic," Hudson said of Bellamy's proposal at the time. She has another son, Ryder, with rocker Chris Robinson.
Though the couple have been engaged for two years, Hudson recently told Elle UK that she and Bellamy have yet to set a wedding date.
"We will get married. I do think it's important, but we have no plans," she said. "[Ryder] wants a party. For me it's not the legal part that is important, it's what it means to the family."