The two stars first met while filming Lumpy in Minnesota and reportedly began seeing each other soon after.
A source close to the couple told Page Six that Timlin and Long have been dating for the last three months.
Reps for both stars did not comment on the report.
Long previously dated his Going the Distance co-star Drew Barrymore and also romanced Gossip Girl's Leighton Meester.
Timlin and Long's film Lumpy, the story of a man dealing with the death of a close friend, is to be released in 2012.
Watch Justin Long and his former girlfriend Drew Barrymore in the Going the Distance trailer below: