The on-off couple, who largely conduct their relationship out of the spotlight, enjoyed a rare red carpet night out at the Berlin Film Festival.
And drawing glances was the golden ring on the Israeli model's engagement finger.
While there's been no confirmation from the couple about future nuptials, a German newspaper is reporting that Leo went down on one knee during Valentine's Day at Berlin's Rome Hotel.
Bar and Leo split last summer amidst reports he wasn't ready to settle down. But if the engagement news is true, the 35-year-old actor has apparently had a change of heart.
Joining the couple at the Cinema for Peace charity gala this week was Leo's German mother Irmelin, who is said to have urged her son to ask for his girlfriend's hand.
A source told Bild newspaper: "She previously urged him to marry Gisele Bundchen without success. Now she said to him, 'Don't make the same mistake a second time!'."