The couple was spotted dining out at the Polo Lounge in Beverly Hills, California on Sunday night (01Apr12) and, according to the New York Daily News, Olsen and the 30 Seconds to Mars frontman couldn't keep their hands off each other.
The 25 year old tenderly embraced Leto and later laid her head in his lap before they left together, reports the publication.
The pair dated in 2005 and was thought to have reunited for a short time in 2008 after Olsen and the singer/actor were seen kissing.
Leto was previously engaged to actress Cameron Diaz and dated Charlize Theron. He was most recently linked to socialite Lydia Hearst-Shaw and Adrian Grenier's ex-girlfriend Isabel Lucas.
Former child star Olsen ended her three-year relationship with actor Justin Bartha last year (11).