He made his debut on 1st October 2010 against the Glasgow Warriors and has made nine European appearances in two seasons.
Marshall came through the ranks at Ulster, graduating from the academy at the start of the 2006-07 season.
He made his full senior squad debut that season against Newport Gwent Dragons and has made 10 European appearances in five seasons, four of which have come in this campaign.
Ulster director of rugby David Humphreys was pleased to announce the re-signing of the duo.
"We have been negotiating to retain Ruan since the summer and are delighted that he has chosen to extend his contract with us," he said.
"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 took his opportunity at the start of the season when Ruan was at the World Cup and has really impressed with his performances, 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."