West Brom winger Chris Brunt says he is "disgusted" and "ashamed" after being struck on the face by a coin thrown by a Baggies supporter.
Brunt had gone over to give his shirt to one of the visiting fans after West Brom lost 3-1 in the FA Cup fifth round at Championship side Reading.
"If people come to football matches and think it is acceptable to throw coins at footballers or anyone else, it is disgusting," Brunt told BBC Sport.
The FA said it would investigate.
"We strongly condemn any such behaviour and will work with both clubs and the authorities to identify those responsible," the governing body said on Twitter.
A Thames Valley Police spokesperson said: "The incident will be dealt with as an assault and we will be working with West Bromwich Albion and Reading to resolve the issue and identify anyone involved."
Meanwhile, the club said it would impose "the severest of sanctions" on the culprit.
The statement released read: "All our true supporters will join the club in condemning the sickening behaviour of a few individuals at the end of our FA Cup fifth-round tie."
Northern Ireland international Brunt has been at West Brom since 2007 and has made more than 300 appearance for the side, but was left to remonstrate with his own fans at the final whistle at the Madejski Stadium.
"I was walking towards our supporters, and they thought it was OK to throw a coin at their own players. It's absolutely disgusting," he said.
"If someone wants to come forward and say who did it, that is not a problem, they know where I am and I am happy to discuss it like normal human beings.
"I hope the police have got hold of them because never in my life have I seen anything like that. I understand frustrations - we haven't played well in the second half and being 1-0 up you shouldn't be losing 3-1."
He added: "You go over to the fans who have travelled down to say thanks for spending their money and a small minority spoil it for them.
"There are some young kids sitting in the front row who have witnessed that and it's not on.
"I wouldn't like to bring my kids to football matches and see something like that."
West Brom manager Tony Pulis said the action was "barbaric", adding: "West Brom is a fantastic football club, a famous club, but you can't condone that.
"I think the police have to get involved in this. I don't think you can allow that to go by. That's criminal."