The glamour model and bodybuilder, who took part in the 2006 series, will set the housemates a series physical challenges in a fitness task, according to The Sun.
The group will be divided into two groups, with the winning team receiving a prize from celebrity chef Aldo Zilli over the weekend.
Marsh hinted on Twitter yesterday that she would be entering the house, telling fans to "keep watching".
Celebrity Big Brother returned to Channel 5 on Thursday with 3.5m viewers.
The housemates won their first task yesterday (January 6), securing Romeo's suitcase in 'Celebrity Circuit'.