The actor and Aryn Drake-Lee have blown off mediation and now face a bitter face-off in court.
The trial is scheduled to begin on September 5 in Los Angeles, according to The Blast.
The pair has been battling over the sum for more than a year, with Williams convinced he's paying his ex too much.
Drake-Lee is fighting to keep the support payments as they are, while claiming he owes her $1 million (£830,000) in retroactive support.
The couple wed in 2012 and split five years later. They share two kids.