The 29-year-old man was detained by Northumbria Police after complaints were received about the racist tweet yesterday.
He is being held in custody pending further inquiries today.
The comment was said to have been posted on the Twitter site following news that United were in the process of signing Senegalese striker Papiss Demba Cisse from German club Freiburg.
The slur is believed to have made reference to the number of black players now in the Newcastle ranks.
Northumbria Police: "We have received a report of racist comments being made on Twitter.
"Northumbria Police takes all reports of racist abuse very seriously and inquiries are being conducted.
"A 29-year-old man has been arrested in connection with the incident and is currently in custody."
NUFC spokesman: "Newcastle United is appalled by the racist comments made on Twitter in reference to the club and its players.
"Newcastle United has a zero tolerance approach to racism and will be working with Northumbria Police to identify those responsible for such abhorrent remarks."