The 36-year-old actress was spotted wearing the ring on her wedding finger at the Chinese Art collection at the LA County Museum earlier this month, where she was pictured with Brad, 48, and their son Pax, eight.
Pitt designed the ring for the mother of his six children in collaboration with jeweller Robert Procop who broke the news of the engagement in a statement last Friday.
He said: 'Brad had a specific vision for this ring, which he realised over a year-long collaboration with Robert,' Procop’s team said in a statement.
'He wanted every aspect of it to be perfect.'
The Hollywood power couple confirmed their engagement last week in a statement - and their six children were said to be especially happy about the news.
Their spokeswoman said: 'It is a promise for the future and their kids are very happy. There's no date set at this time.'
Procop said: 'Brad had a specific vision for this ring, which he realised over a yearlong collaboration. He wanted every aspect of it to be perfect, so I was able to locate a diamond of the finest quality and cut it to an exact custom size and shape to suite Angelina's hand.
He continued: 'Brad was always heavily involved, overseeing every aspect of the creative design evolution.
'The side diamonds are specially cut to encircle her finger. Each diamond is of the highest gem quality.'
Experts estimate the ring cost around $250,000.
Angelina's father Jon Voight was among the first to publicly congratulate the pair, telling Us Weekly: 'I'm very happy for them.'
Angelina has been married twice before - to British actor Jonny Lee Miller between 1996 and 1999 and then Billy Bob Thornton, from 2000 to 2003.
Brad's marriage to Jennifer Aniston lasted five years before they announced they were divorcing in early 2005 - which came after he had met Angelina on the set of Mr & Mrs Smith.
Brangelina - as they became known - quickly became a Hollywood power couple, with a brood of six children; five-year-old Shiloh and twins Knox and Vivienne, aged three, along with three adopted children Maddox, 10, Pax, eight, and Zahara, seven.
The former Friends star is said to be delighted for the couple.
The actress, 43, is so happy with her own beau Justin Theroux that she is actually pleased with the situation, according to reports.
According to HollywoodLife.com a source said: 'Jennifer is happy for Brad and Angelina. She is so in love with Justin, that she is really grateful how things turned out.
'In a strange way if it weren’t for Angelina, Jennifer would not have connected with Justin in a romantic way. Jennifer is in a really wonderful place right now.'
The couple once insisted they wouldn't wed until the legalisation of same-sex marriage in America.
But Brad recently admitted he didn't feel they'd be to wait that long because of mounting pressure from their children.
'It seems to mean more and more to our kids,' he said in January.
'We made this declaration some time ago that we weren't going to [get married] till everyone can. But I don't think we'll be able to hold out … it means something to me, too, to make that kind of commitment.'
She said just months ago: 'We've explained to them that our commitment, when we decided to start a family, was the greatest commitment you could possibly have.
'Once you have six children, you're committed.'
Their high-profile romance has made headlines since sparks first flew on the set of Mr & Mrs Smith back in 2004.
There was intense speculation they started up a relationship while Brad was still with his then-wife Jennifer Aniston.
Brad and Jen announced they were divorcing in early 2005, but it wasn't until early 2006 that Angelina and Brad acknowledged their relationship when the actress revealed she was pregnant.
In October 2008, Angelina told The New York Times that her children loved Mr & Mrs Smith because: 'Not a lot of people get to see a movie where their parents fell in love.'
But she has explained: 'To be intimate with a married man, when my own father cheated on my mother, is not something I could forgive. I could not look at myself in the morning if I did that. I wouldn't be attracted to a man who would cheat on his wife.'
