Mississippi has become the fourth state to allow legal sports betting as of Wednesday, according to David Purdum of ESPN.
Sportsbooks at Gold Strike Casino in Tunica and Beau Rivage in Biloxi were the first to begin accepting wagers.
The state joins Nevada, Delaware and New Jersey as the only states to legalize the practice, although Mississippi becomes only the second after Nevada to allow betting on in-state colleges.
The Supreme Court ruled states were allowed to make their own laws on the practice in May, which led to Delaware and New Jersey becoming the first to legalize it.
However, both prevented wagers on in-state colleges such as Rutgers. Mississippi has no such restrictions, allowing fans to bet on Ole Miss or Mississippi State during the upcoming football season.
Mississippi had initially legalized sports betting in 2017 to help allow wagering on daily fantasy sports, but official regulations went into effect in July.
According to Purdum, $750 million to $800 million is expected to be wagered annually at casinos within the state.
Former NFL running back Willis McGahee was among the first to place a bet during Wednesday's opening.