Dre Greenlaw's sideline headbutt has cost him a game and an NFL check after being sanctioned for his actions on Sunday.

The NFL suspended the San Francisco 49ers linebacker without pay for one game on Monday for unsportsmanlike conduct in Sunday's 32-29 win over the Seattle Seahawks, with the NFL using a barely-known rule to eject the star linebacker. Greenlaw is appealing the suspension, sources told ESPN.

If it stands, he will miss the Oct. 19 game against the Washington Commanders and can return to the active roster on Tuesday, Oct. 20. The incident happened in the second quarter. Greenlaw chased quarterback Sam Darnold as he scrambled toward the Seattle sideline, and Darnold stepped out of bounds before taking a hit from Greenlaw.

Greenlaw was flagged for the late hit, and several Seahawks surrounded him. Linebacker Ernest Jones IV confronted Greenlaw and appeared to push his facemask.

The 49ers star responded by using his helmet to headbutt Jones, who was not wearing one. Jones was bleeding after the contact, according to AP. The league said Greenlaw violated Rule 12, Section 3, Article 1, which covers "any act which is contrary to the generally understood principles of sportsmanship."

Greenlaw was ejected under a new rule that allows the league's Art McNally Gameday Central in New York to disqualify a player even without a flag. The review cited him for non-football conduct, according to the postgame pool report.

Any appeal would be heard by one of four hearing officers jointly appointed by the NFL and the players' association: Derrick Brooks, Jordy Nelson, Ron Rivera, or James Thrash.

This is not Greenlaw's first run-in with the league. It was his third career ejection, after a late hit on Justin Herbert in 2022 and a sideline dustup with the Philadelphia Eagles in 2023.

He was suspended for one game in 2025 while with the Denver Broncos after he ran toward referee Brad Allen and, according to the league, "verbally threatened him." Last week, he was fined $11,941 for unsportsmanlike conduct after he repeatedly yelled at the Broncos' sideline and coach Sean Payton.

Coach Kyle Shanahan said he had not spoken with Greenlaw yet but planned to do so. "We just celebrated there," Shanahan said. "We'll clean that stuff up. He knows he's got to be smarter."

Linebacker Fred Warner pointed to the situation Greenlaw was in. "He's all by himself on their sideline," Warner said. "Of course, they're trying to gang up on him, and good on them for baiting him into doing something. But we know we got to be smart in all those situations."

Rookie Jaden Dugger moved to the weakside spot in Greenlaw's absence and finished with nine tackles and a sack. "Dugger did a great job stepping it up," Shanahan said.

The 49ers improved to 5-0 with the win, and Mike Evans starred in the fourth quarter, helping set up Eddy Pineiro's 43-yard go-ahead field goal with 1:56 left.