The actor, one of Britain’s best-known bachelors, is to tie the knot with Swedish girlfriend Anna Eberstein, 39, later this month.
Grant played commitment-shy characters in comedy films Four Weddings And A Funeral and About A Boy.
The leading privacy campaigner, who wants to gag Press freedom, has had a number of high-profile relationships in real life.
The engagement emerged in wedding banns posted at Kensington and Chelsea register office, near the couple’s £10million home in West London.
The official notice of the forthcoming marriage was on display on screens in the office at Chelsea Old Town Hall.
A source said: “This is a day none of Hugh’s friends ever thought would come.
“He will soon be a pensioner and has finally become a family man.”
The star, who has three kids with Anna and two with former flame Tinglan Hong, has dated a string of women.
Best known was his 13-year romance with actress and model Liz Hurley.
They split in 2000, five years after he was caught with Hollywood hooker Divine Brown.
Grant also dated socialite Jemima Khan for three years before she reportedly dumped him in 2007.
The actor was romantically linked to film co-stars Martine McCutcheon and Drew Barrymore, as well as posh PR girl — and former squeeze of Prince Andrew — Caroline Stanbury.
But after meeting retail director Anna in a London bar six years ago, Grant finally appears to have settled down.
Until now, it was always thought he would be an eternal bachelor — or, as he once put it, “a sad, lonely old man”.
Grant, who plays disgraced MP Jeremy Thorpe in A Very English Scandal, which starts on BBC1 tonight, said two years ago marriage was “unromantic”.
He added: “Do I think humans are meant to be in 40-year-long faithful relationships? No, No, No.”
But best pal Hurley says fatherhood has changed him.
She said in March: “Having these kids has transformed him from a very miserable person into a fairly miserable person.”