Pienaar has played in the number 10 jersey for his previous club Natal Sharks, as well as for the Springboks. The Ulster back-line remains otherwise unchanged from the one that faced the Cardiff Blues last weekend.
Up front, Pedrie Wannenburg has returned from a foot injury and will pack down at number 8 in place of the benched Robbie Diack.
Chris Henry will again captain the side from the blindside flank, with Willie Faloon remaining on the other side of the scrum.
Dan Tuohy and Lewis Stevenson continue the partnership in the second row. There are two changes in the front row with Paddy McAllister coming in at loosehead prop for Callum Black and Andi Kyriacou replacing Nigel Brady at hooker.
There are still tickets available for Friday's game. They can be bought online at www.ulsterrugby.com or by calling the ticket office on 02890493222. Alternatively, you can pay at the gate on match night.
Ulster Rugby are advising supporters coming to the game (gates open 5.30pm) to avoid parking in the immediate vicinity of the Ravenhill grounds. Park 'n' Ride facilities are available at Pirrie Park and along Montgomery Road.
ULSTER (v Ospreys): Stefan Terblanche; Craig Gilroy, Nevin Spence, Paddy Wallace, Ian Whitten; Ruan Pienaar, Paul Marshall; Paddy McAllister, Andi Kyriacou, John Afoa, Lewis Stevenson, Dan Tuohy, Chris Henry (capt), Willie Faloon, Pedrie Wannenburg.
Replacements: Nigel Brady, Callum Black, Adam Macklin, Tim Barker, Robbie Diack, Ian Humphreys, Simon Danielli, Adam D'Arcy.