The 32-year-old actress gave birth to a 'healthy baby boy', according to her publicist, in Los Angeles on Saturday. The couple's name choice for their baby has not been announced yet.
Kate's new son is her second child - she already has a son Ryder, seven, from her first marriage to Black Crowes singer Chris Robinson. They divorced in 2007.
Kate and Muse front man Matt, 32, began dating last year. Kate had said as she 'already had one child and one divorce' she didn't see the point of getting married again but Matt proposed to the pregnant actress in April.
Kate is Goldie Hawn's daughter. We're wondering whether Goldie is the most glamorous grandmother ever.
Congratulations to Kate and Matt!