The Hollywood star, who already has an eight-year-old son, had previously spoken of a “hunch” she was carrying a girl.
But Kate, 32, and Muse frontman Matthew Bellamy had a healthy son on Saturday at a Los Angeles hospital.
Hudson’s rep confirmed the happy news to People magazine but didn't give details about the tot's weight or name.
In January, the Fool’s Gold star had announced she and Bellamy, 33, were expecting a baby.
The two got engaged in April after dating for about a year.
Hudson - daughter of Goldie Hawn - had insisted she didn’t want to know the sex of the child in advance, but had a feeling it was a girl.
Hudson has son Ryder Russell with ex-husband Chris Robinson, frontman with the Black Crowes. This is Bellamy's first child.