|Kristen Stewart at the Snow White premiere
Kristen, 22, who plays Snow White, presented the film with the cast on stage while Robert watched from the audience.
A source told Us Weekly: "Rob was Kristen's date. Rob was cheering and clapping for Kristen as they walked off stage. He looked proud of his girlfriend."
After the premiere, the cast attended Ciroc Ultra Premium Vodka's official after party at the Royal Courts of Justice.
The actress - who stars as Bella Swan with her real-life partner's Edward Cullen in the vampire movies - changed out of her black lace Marchesa gown into a leather jacket and jeans.
Apparently, Robert - who celebrated his birthday Sunday - also dressed casually in his trademark baseball cap.
An insider added: "Rob met Kristen there. They stayed together in the VIP area all night and looked really cozy. They made sure no one saw them or photographed them."
The publication reports that he also brought his family to the event.