Charlize Theron accompanied her longtime boyfriend Sean Penn to the premiere of his new movie The Gunman in London at BFI Southbank in London Monday.

Charlize Theron At ‘The Gunman’ Premiere

While Penn is the star of the movie, Theron stole the show on the red carpet in a black halter Halston Heritage gown with a black belt with a gold buckle fastened at her waist. The Mad Max: Fury Road actress finished off her look with Stuart Weitzman sandals. At her side, Penn looked sharp in a navy blue suit and tie.

Penn has been dating Theron since early 2014, when their 18-year friendship took a romantic turn.

“If you have that kind of enduring friendship like we did, you value it and you don’t want to just make an impulsive decision that can damage that,” Theron revealed on The Today Show. “You always kind of have to evaluate or re-evaluate your actions moving forward.”

When asked about the relationship, Penn has been effusive in speaking about his love for Theron, even diminishing his past marriages – to Madonna and Robin Wright – in the process.

“I’m surprised to be in love. Lot of reasons. 53-years-old plus finally beginning to figure out why you haven’t been happy in a single relationship? It could seem too late,” Penn said. “To run into somebody now who you care about is a much passionate, deeper, truer and – God! – a much happier feeling. It’s a lot more romantic and a lot more fulfilling to be in a relationship and to think you’re a good person within it.”

The Gunman, in which Penn plays a military contractor suffering from PTSD, hits theaters in the U.S. March 20.