Sienna, who apparently split from the Brothers And Sisters star last month, arrived in Positano with Balthazar's mum on Friday.
The small town on the Amalfi coast holds special significance for Sienna and the dad of four, as it's where they cemented their relationship last year.
Balthazar made an appearance on Saturday, when the couple took a boat together to Nerano. And they looked closer than ever, kissing and cuddling during a lunch with a group of Balthazar's family and friends.
Fans of the actress may be surprised by her about turn, after her admission last week she faced criticism for getting together with Balthazar. She also said she was still in love with former fiancé Jude Law.
Balthazar is separated from his fashion designer wife Rosetta Millington, with whom he has four children. While there's been no mention of a divorce, his reunion with Sienna could change things.