NBA rules require that any player who deliberately enters the spectator stands during a game be automatically ejected and subject to a fine and/or suspension, the league said in its announcement Sunday.
Thomas was ejected late in the fourth quarter of Saturday's 125-108 loss in Philadelphia after going into the stands and verbally confronting a fan at Wells Fargo Center.
The fan and another spectator who was also involved in the confrontation have been banned from the Wells Fargo Center for a year.
Thomas said he was running back after making one of two free throws and that a fan had "both of his middle fingers up and said, 'F--- you, b----,' three times."
"I said: 'Don't be disrespectful. I'm a man before anything. Be a fan.' His response was, 'I'm sorry, I just wanted a Frosty,'" Thomas said, adding that he believes the fan's behavior stemmed from a 76ers promotion in which fans are awarded a frozen dessert if an opposing player misses two free throws in the second half.
The league noted in its announcement that the automatic ejection of a player who enters the stands is intended to prevent altercations or other hostile interactions between players and fans and is therefore enforced even in circumstances such as the encounter between Thomas and fans, which did not escalate.
Thomas appeared to express his displeasure with the ruling Sunday night, tweeting "Bulls---," shortly after the decision was announced.
He will begin serving his suspension when the Wizards visit the New York Knicks on Monday.
In October, the NBA instituted a zero-tolerance policy for abusive or hateful behavior by fans. The new policy followed high-profile incidents involving Russell Westbrook, DeMarcus Cousins, Kyle Lowry and others during the 2018-19 season.
The Sixers fans who were banned acknowledged using inappropriate gestures and language toward players, which is a violation of arena policy and the NBA code of conduct.
The season-ticket holder for the seats has had them revoked and will receive a refund for the remaining games, the 76ers said.
Thomas ejected for entering stands
Isaiah Thomas is ejected from the game for an incident between him and two Philly fans that led him to go into the stands.