The star of the film Dukes of Hazzard and the Panic Room star have been spotted displaying their affections for one another.
The two artists have been together for the past few days enjoying time at several Los Angeles night clubs.
Simpson, 25, is best known for her reality television series Newlyweds: Nick and Jessica, with her ex-husband, Nick Lachey. Simpson filed for a divorce in December.
Leto, 34, has previously been involved with actresses Cameron Diaz and Scarlett Johansson.
Representatives for the two stars have not confirmed that the Simpson and Leto are a couple.