He is being held at Swansea Central police station, South Wales Police said.
The comments appeared on the Twitter account @liamstacey9, which has since been taken down.
While the feed was still visible comments claimed that his account had been hacked.
Other users of the micro-blogging service responded with outrage to the remarks.
Muamba, 23, was rushed to hospital after suffering what appears to have
been a heart attack on the pitch before 36,000 horrified spectators last
He is currently in critical condition in hospital.
The footballer's illness has prompted a wave of support and prayers from both fans and others in the world of football.
A police spokesman said: "South Wales Police have arrested a 21-year-old
male from the Pontypridd area
In: Regional News
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