Miller stars in “The Lost City of Z” alongside Charlie Hunnam, Robert Pattinson and Tom Holland.
It seems that their tête-à-tête has perked up the actor, who has been looking seriously thin during his divorce from Angelina Jolie.
It seems to be the first public outing for the significant piece of jewellery, as Sienna wasn't spotted wearing the ring at a fashion party last week.
Mail Online has contacted a representative for Sienna for comment.
It was claimed that the Hollywood stars had been trying to keep their dates a secret, according to new reports, but their cover was seemingly blown when they were seen looking very intimate in a Glastonbury VIP area.
Despite the reports, sources told Mail Online: 'She's happily with Bennett and has no idea where these rumours keep coming from.'They're just not true. They were hanging out at the Rabbit Hole at Glastonbury but they were with a big group.' When the star first soared to prominence with her role in Alfie, she soon fell for her co-star Jude, who she became engaged to in 2004.