Thomas recovered a fumble by New Orleans Saints running back Mark Ingram in the first quarter and raced down the right sideline for a 34-yard touchdown. He gave the football to a fan in the stands, then ran toward an official with his arms wide open before going in for the hug.
The official, side judge Alex Kemp, didn't seem to appreciate the gesture and reached into his pocket immediately to flag Thomas for an unsportsmanlike conduct penalty.
The 15 yards were enforced on the ensuing kickoff.
Thomas didn't realize he was flagged for the hug until after the the Seahawks' 25-20 loss to the Saints.
"I just found out they penalized me for that," he said while sitting at his locker.
"I'm having fun, just excited about the moment. I only live three hours away from here. I'm just thinking, like, 'Man, everybody back home just saw me score.' You never know what happens when you go into these games, and then the ball goes your way. Just an exciting moment. That's just how I celebrated."
The NFL rulebook, in addressing contact with officials, states, "Under no circumstances is an official to allow a player to shove, push, or strike him in an offensive, disrespectful, or unsportsmanlike manner. The player shall be disqualified from the game, and any such action must be reported to the Commissioner."
ESPN's Kevin Seifert contributed to this report.