Liam Stacey, who is studying at Swansea University, today admitted a racially-aggravated public order offence at Swansea magistrates court.
He was arrested after comments he made on Twitter on Saturday following Muamba's collapse in an FA Cup tie were reported by other users of the social network.
Stacey has been released on bail and is scheduled to be sentenced on March 27.
He posted the offensive comment on the @liamstacey9 page shortly after Bolton Wanderers midfielder Muamba collapsed on the pitch, and was thought to have died.
Following the tweet, a Twitter user sent Stacey a message saying: "Your sick tweets have been passed on to the Police."
Another user contacted the @SwanseaUni page to ask: "Are you aware your student @LiamStacey9 has been tweeting racist abuse regarding the serious injury of a black man?"
The court heard today how police forces from all across Britain had received complaints about the comments.
Stacey initially attempted to claim that his account had been hacked, but later tried to delete the page. He was arrested on Sunday outside his house in Swansea.
BBC News reports that he told police that he had been drinking heavily on Saturday and was drunk when he made the comments.
Stacey told police: "I was at the bar when I heard what had happened to Muamba. I don't know why I posted it. I'm not racist and some of my friends are from different cultural backgrounds."
The student was said to be close to tears as the court heard the evidence against him, including that he had texted a friend admitting to the comments.
In the SMS, he wrote: "I said something about Muamba that I shouldn't have and tweeted back to some people who abused me. Getting police on me now which isn't good at all."
Stacey was warned that he could face jail time, and must not use social networks including Twitter until he is sentenced.
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