The two apparently have been friends since childhood, when Ali's father Dustin helped Hill land his first major role in I Heart Huckabees.
A source close to the couple claimed that Hill and Ali's relationship recently turned from friendship to romance and they are now an item, according to Page Six.
The insider added that Hill and Ali even spent Christmas together at the Hoffman family's vacation home in Hawaii.
Hill's spokesperson has not commented on the report.
The actor reportedly split from his longtime girlfriend Jordan Klein late last year because of their busy schedules.
Hill celebrated his 28th birthday earlier this week at a star-studded party attended by Brad Pitt, Angelina Jolie and Zooey Deschanel.
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Watch Jonah Hill in the Moneyball trailer below: