The shiny new sparkler on her ring finger seems to suggest the couple is ready to take the next step after dating for six months.
Theron, 38, was photographed wearing new bling while walking through Los Angeles International Airport on Wednesday, the New York Daily News reports.
But a source tells PEOPLE that the 18-karat yellow gold Anita Ko leaf ring (which is valued at $4,900), was a gift, and the Oscar winners aren't engaged.
"[Charlize] likes guys with more substance. She finds Sean very charming and he makes her laugh," a source close to her tells PEOPLE. "She definitely has him wrapped around her finger."
Reps for both Theron and Penn had no comment.
The actors have been dodging proposal rumors ever since they got together in January, making their first public appearance at Penn's third annual Help Haiti Home gala. Friends for years, sparks flew between the stars during vacations to Hawaii and Mexico.
One insider told PEOPLE in April that the pair's pals "wouldn't be surprised" by a trip down the aisle.
Next, they'll take their chemistry to the set of The Last Face, directed by Penn and featuring Theron.