Following plenty of speculation during the pre-season, Marcelo completed his move to The Buccaneers on deadline day last Friday.
The club had been weighing up their final foreign registration spot for the season, and have finally decided on the 20-year-old centre-back.
He was initially invited for a trial at the Mayfair-based outift, after he was spotted during the Project X development tour of Brazil earlier this year.
"Orlando Pirates Football Club is delighted to announce the signing of Caio Marcelo from Nova Iguaçu Football Club," read a club statement.
"The signing of the Brazilian defender was concluded on the final day of the transfer window on Friday last week."
The young defender, who was also previously on loan at Brazilian Serie A side Vasco Da Gama, is set to strengthen the central defence as a rare left-footed option for coach Milutin Sredojevic.