The 20-year-old Italian, who is on loan at Queens Park Rangers, was charged by the Football Association last month along with Newcastle forward Nile Ranger and Walsall's Manny Smith.
An FA statement today read: "Federico Macheda has been fined £15,000 and warned as to his future conduct after the Manchester United player admitted use of an abusive and/or insulting word including a reference to a person's sexual orientation.
"Macheda had requested a personal hearing in relation to the charge for comments he made on Twitter."
Ranger and Smith were last Thursday fined £6,000 and £1,200 respectively and also handed a warning by the FA after admitting making homophobic comments on Twitter.
West Ham midfielder Ravel Morrison was also recently fined £7,000 by the FA in response to a homophobic remark made on Twitter.
Morrison, who completed an undisclosed switch to Upton Park from Manchester United at the end of January, is reported to have made the comment in response to another Twitter user.
The 19-year-old was also warned as to his future conduct by the FA.