After completing the 22-yard throw to Emmanuel Sanders, Roethlisberger jumped into the arms of tackle Flozell Adams. Roethlisberger then began walking forward and appeared to say something to Seymour, who turned and struck the quarterback in the jaw with his open right hand.
As Roethlisberger went to his back, lineman Chris Kemoeatu grabbed Seymour. Both Seymour and Kemoeatu drew penalties, but only Seymour was ejected.
After the game, Seymour told the media he was sorry for the outburst.
"I apologize to my teammates," Seymour said. "I never want to do anything to hurt the team. You always want to protect yourself, but there's no excuse."
Seymour said he was confused by Roethlisberger approaching him after the score.
"I'm not sure why he [Roethlisberger] ran up on me. I just turned around and it was a natural reaction," Seymour told media after the 35-3 loss.
Seymour said he doesn't remember Roethlisberger saying anything to him and Roethlisberger said afterwards he was stunned by Seymour's actions.
"I just said let's get ready for the extra point. It blew my mind," Roethlisberger said. "We knew coming in that it was going to be a physical game. There was a lot of pushing and shoving and extracurricular stuff. It was a physical-natured game. I remember trying to get between guys, 'Let's move on, we scored, let's go.'"
Steelers coach Mike Tomlin said it wouldn't change his opinion of the 11-year veteran.
"I haven't seen a quarterback get punched since I've been in this league after a play like that. It was unfortunate," Tomlin told the media after the game. "I got big-time respect for Richard Seymour as a football player. That guy's got an 11-year resume that's pretty impressive as a professional ... I'm not going to let that play cloud my opinion of Richard Seymour."
Information from The Associated Press contributed to this report.