Photos printed in UK newspapers showed the pair posing for pictures on the steps of Chelsea register office on Friday with a small group of family members.
Grant, 57, was seen in wearing a dark blue suit, while Eberstein, 39, was dressed in a blue shirt and white skirt, and wearing a simple gold wedding band.
Grant’s publicist has not responded to media requests for comment.
Eberstein is the mother of three of Grant’s children.
In 2012, the couple had their first child together – a son.
The pair then had a daughter, whose name has not been revealed, in December 2015.
In March this year, Grant’s ex-girlfriend Liz Hurley revealed Grant and Eberstein had recently welcomed a third child but the sex is unknown.
Grant had two children – Tabitha and Felix – with former partner Tinglan Hong.
The actor also dated Jemima Khan in the past.
Grant, who has played a string of commitment-phobic characters in films such as Four Weddings And A Funeral and Love, Actually, was once well-known for his reluctance to marry.
In 2015 he told People magazine: “I’m not really a believer in marriage. I’ve seen very few good examples, maybe five, in my life, but I think otherwise it’s a recipe for mutual misery.”