The `Revolutionary Road` actress - who has daughter Mia, 12, with first husband Jim Threapleton and son Joe, nine, with second spouse Sam Mendes - and partner Ned RocknRoll tied the knot in an intimate ceremony in New York earlier this month.
A source told The Sun newspaper: "It was a really romantic, private ceremony."
Just 12 of the couple`s friends, including the 37-year-old actress` `Titanic` co-star Leonardo DiCaprio - who gave the bride away - were in attendance at the wedding, which took place in a converted barn, and even their families were unaware of the nuptials.
A source said: "Kate and Ned got married in a really romantic and private ceremony. They had been planning it for a while. They are both so happy and very much in love. Kate is very private and didn-t want anyone to know.
"She asked if Leo would give her away because they are best friends."
Kate`s spokesperson has confirmed she has marriedagain, but when her father Robert was asked about the wedding, he said: "What are you going on about?"
Ned`s family were equally shocked, with his father Robert saying: "It`s not something I know of - but nothing would surprise me with those two."
Kate and 34-year-old Ned - who has changed his name from Abel Smith - got engaged this summer after a year of dating.
A friend said: "Kate has never looked so happy. Since she and Ned got together it has been full steam ahead.
"Marriage has always been on the cards and they`ve been talking about doing it for the last couple of months."
Ned is the nephew of tycoon Sir Richard Branson, and works part-time for his space venture Virgin Galactic.