The transfer does come as a surprise given that The Natal Rich Boys had been adamant that Mbuthuma, fresh from signing a five-year contract at the club, would not be for sale at any price.
But that has quickly changed with the public broadcaster being informed that the 23-year-old has put pen to paper with the Soweto giants.
He is the second player from Richards Bay to join Pirates during the current transfer window after Nkosikhona Ndaba, 22, was snapped up a few months ago.
It may very well have been a package agreement between the KwaZulu-Natal outfit and their Gauteng rivals, however that is not immediately clear.
Mbuthuma impressed Pirates with his form this season, which was also amplified by the six goals in 27 matches from all competitions despite Richards Bay initially struggling in the early stages of the season.
They were eventually able to pick up momentum and finish in the top eight, subsequently offering their best assets long-term contracts in an attempt to discourage potential suitors.
However, it is understood the Pirates offer was too good to refuse and Richards Bay have already signed Gabadinho Mhango after the veteran Malawi international parted ways with Marumo Gallants.