It was the first chance for celeb fans to get a good look at the impressive sparkler, which Amber has kept hidden on other outings.
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Diamond experts have estimated the ring to be a five-carat, oval diamond which could be valued at around the $100,000 mark.
The huge bauble glittered on Amber's engagement finger as she posed with her fiancé at the premiere of 3 Days to Kill.
Johnny looked happy to be supporting the actress at the unveiling of her new project. The notoriously private star even put on a rare public display of affection – at one point kissing Amber tenderly on the cheek.
For the evening out the 27-year-old actress chose a low-cut black dress featuring a daring side split which showed off her toned legs. She finished the look with gold shoes and red lips and nails.
Her blonde hair, meanwhile, was backcombed and swept over to one side – giving a glimpse of her delicate diamond drop earrings.
Johnny smiled for the cameras as the pair stood side by side at the ArcLight Hollywood. And as they made their way inside, he took Amber by the hand.
Johnny's representative confirmed last month that the pair intend to marry. According to sources they will tie the knot on the Hollywood star's private island in the Bahamas.
The pair first met in 2011 when they starred together in The Rum Diary. It wasn't until the beginning of 2012 that they were linked romantically after the breakdown of Johnny’s 14-year relationship with Vanessa Paradis, with whom he has two children, Lily Rose,14, and Jack, 11.
Johnny, 50, reportedly called Vanessa to tell her that he had proposed to his Rum Diary co-star and the 41-year-old gave the couple her blessing.
"There's no drama," a source told US Weekly. "Vanessa wished him happiness. She's not bitter, she's happy."