"He has been instrumental in the team's success since joining us in 2010. He's a true match winner and a total professional from his attitude in the gym, through the mentoring role he plays with younger players to the quality of performance he delivers consistently each week.
"Paul (Marshall) took his opportunity at the start of the season when Ruan was at the World Cup and has really impressed with his performances.
"That was evident last week when he was involved in three of the four tries Ulster scored against Edinburgh. Paul has made a huge contribution to the squad in the last few years both on and off the field.
"Scrum half is such a key position within the team that every squad needs two quality players in this position and we are delighted that both have chosen to remain with Ulster Rugby."
Pienaar was part of South Africa's 2011 Rugby World Cup squad. He has been capped 51 times for the Springboks and made his debut during the 2006 Tri Nations against New Zealand.
Born in Bloemfontein and the son of former Springbok fullback Gysie Pienaar, he joined Ulster on a two-year contract at the beginning of the 2010/11 season from the Natal Sharks.
He made his debut for the province in October 2010 against Glasgow Warriors, scoring all of their points in a 19-17 win over the Scottish side.
At the end of his first season in Ulster colours he not only collected the Player of the Year award at the end of season Heineken Ulster Rugby Awards, but also scooped the Magners League Player of the Year award, voted for by his peers. Pienaar was also named in the Magners League Team of the Year.
He said: "I've really enjoyed my first two seasons here when and the opportunity came to extend the deal I was very excited.
"I've made some good friends here and I've really enjoyed the rugby side. The people of Belfast have welcomed myself and my family with open arms and so it was an easy decision in the end."
Marshall joined Ulster on a full contract at the start of the 2006/07 season having graduated from the Ulster Academy.
He made his full senior debut that season against the Newport Gwent Dragons, although had previously played with the Ulster seniors in the pre-season friendlies of 2005/06. A former pupil of Methodist College, he has international honours at Under-21, 'A' and Sevens level.
His lively play around the fringes at ruck time make him a constant threat to the opposition and his defensive work is another key attribute of his game, despite being the lightest player in the squad.
Marshall commented: "I think with the way things have been going the last number of weeks we're really moving forward.
"When you look at the strength of our squad and the players we're signing, it's showing a real intent from the management right the way down that we want to compete with the best teams in Europe and be the best team in Europe.
"From my point of view I am happy to be here and I'm really enjoying my rugby at the minute.
"Away from rugby I have so much going on that is so important to me and it's great that I can play rugby professionally and do it in my home town.
"I think that's a really privileged position to be in and something that I would hope never to take for granted."