Midfielder Jordan Henderson said fans were being "misled" by bogus pages
Sunderland Football Club has warned fans about "imposters" posing as star players on social networking web sites
Fake profiles claiming to be those of Black Cats players have appeared on Facebook and Twitter, the club said.
Midfielder Jordan Henderson said he was "horrified" when he discovered that someone was posing as him on a social networking site.
The club said it feared those posing as players could influence or even make contact with fans, including children.
The club has stressed that no first team players have any official player profile pages on any social networking sites.
Jordan Henderson, said: "It's horrible to think that supporters are being given wrong information about players or that they are receiving misleading messages from people claiming to be us.
"It's really difficult to know who is actually behind the profiles so you don't know who you're giving your personal information to and what they could be doing with it."
Steve Walton, SAFC's chief executive, said: "We have noticed a steady increase in impostors creating personal profiles claiming to be Sunderland players.
"These unknown individuals are continuing to communicate and interact with supporters, including a high proportion of school children.
"A fundamental part of any professional football club is to protect supporters."
Mr Walton has urged fans to "err on the side of caution" and to not interact with profiles pretending to be players.