The league said he will serve the suspension Friday night when Boston hosts the Cleveland Cavaliers.
Williams was given a technical foul and then ejected in the fourth quarter of the Celtics' 120-102 loss at Chicago on Monday night after he was called for an offensive blocking foul while defending Zach LaVine.
He jumped in protest, aggressively walking away from the court and over to the sideline just in front of the first row of fans. As he walked over to the fans, he also made what crew chief Marc Davis told a pool reporter after the game was "intentional physical contact" with referee Cheryl Flores.
The Associated Press contributed to this report.