The NFL handed Mayfield a fine of $10,026 for unsportsmanlike conduct, the result of an apparent lewd gesture he made toward Cleveland Browns offensive coordinator Freddie Kitchens in Sunday's 26-18 win over the Cincinnati Bengals.
Mayfield threw a 1-yard touchdown pass to tight end Darren Fells in the second quarter, then went to the sideline and gestured toward his groin area.
Asked about it this week, Mayfield said he couldn't recall what happened.
"I honestly couldn't tell you," he said. "I don't know. We have a lot of stuff within the locker room, what we have going on in the offense. Lot of inside jokes. Who knows what it was?"
A league source said the NFL office looks on Mayfield's actions seriously.
"Suggestive gestures have long been prohibited and are prohibited anywhere on the field," the source said. "Sportsmanship is something we take very seriously, especially as it relates to setting a good example for athletes at other levels of the sport."
Kitchens said he had no problem with Mayfield's actions.
"I don't understand what he did wrong, what the big deal is," Kitchens said earlier this week. "He's in middle of a football game in the NFL. I like guys that have a relentless pursuit of competition. I don't have a problem with Baker."
Mayfield's agent told Cleveland.com that the quarterback will appeal the fine.
The quarterback's base pay is $480,000, or $28,235 per week, meaning the fine is almost 36 percent of his weekly pay. This is the first fine of Mayfield's NFL career.
Information from ESPN's Pat McManamon was used in this report.