Earlier this week, it was reported that the couple had split after three months together, although no reason for the break-up was given.
However, Penn and Johansson were pictured together at last night's ceremony, suggesting that the relationship is still going strong.
Penn was attending the event as part of a reunion with the cast of the 1982 coming-of-age drama comedy Fast Times at Ridgemont High, which was inducted into the Guy Movie Hall of Fame.
Johansson split from fellow actor Ryan Reynolds last year, and the two divorced soon after.
The actress was soon linked to Penn, although the couple only made their first public appearance together in March at Reese Witherspoon's wedding.
Watch Sean Penn in the trailer for The Tree of Life below: