Ryan Babel has been fined £10,000 and warned about his future conduct for posting comments and a doctored picture of Howard Webb on Twitter.
The Liverpool forward was charged with improper conduct by the Football Association after he used the social networking site to criticise the official.
After the Reds' FA Cup exit at Manchester United, where Liverpool conceded a controversial first-minute penalty and had captain Steven Gerrard sent off, Babel posted a mocked-up picture of Webb wearing a United shirt.
He also wrote: “And they call him one of the best referees? That's a joke. SMH (shaking my head).”
Babel later apologised for his comments and posted: “My apology if they take my posted pic seriously. This is just an emotional reaction after losing an important game.
“Sorry Howard Webb.”
At a regulatory commission hearing the 24-year-old was fined £10,000 and warned as to his future conduct.
Regulatory commission chairman Roger Burden said: “Social network sites, like Twitter, must be regarded as being in the public domain. All participants need to be aware, in the same way as if making a public statement in other forms of media, that any comments would be transmitted to a wider audience.
“It is their responsibility to ensure only appropriate comments are used.”
Professional Footballers' Association chief executive Gordon Taylor said: “Players have got to be mindful of what they say and make sure that it's in the rules of the game and not bringing the game into disrepute.”