The 39-year-old actress recently declared as much in an interview for the June issue of Elle U.K. Penn previously said the same about the star while speaking to French newspaper Le Parisien in February.
"Putting aside that he's my partner, he's the love of my life," Theron asserted before detailing their trying collaboration on Penn's film The Last Face. "For the first time, I felt that my work was really bleeding into my life, and that made it hard."
"There were moments where I was incredibly unfair to him," she admitted. "And moments where I felt like... he was incredibly unfair to me. But it makes you realize that no matter how complicated it gets, the priority is the relationship."
Theron and Penn were longtime friends before being linked in January 2014. The couple confirmed their relationship in May, and the actress said she is open to the possibility of marriage to Penn in an interview with Esquire published last month.
Theron was previously involved with actor Stuart Townsend for nine years, and adopted her son Jackson in 2012. Penn was wed to singer Madonna from 1985 to 1989 and actress Robin Wright from 1996 to 2010, and shares daughter Dylan and son Hopper with Wright.