The 32-year-old graduated from the Academy to a senior Ulster contract in 2006, making his debut for the team against Newport Gwent Dragons in September of that year.
A familiar face to the Kingspan Stadium faithful, Marshall has amassed 204 caps for the Province, scoring 24 tries. He has also earned three Ireland caps.
Commenting on his decision, Marshall said:
“Ulster Rugby has been a massive part of my life and I’m grateful to have been involved with the squad for the last 12 years.
“There’s no doubt that I will be sad to leave as I love the club and the people here. I would love to continue playing, but there comes a point for everyone to move on and seek new challenges and that time has arrived for me.
“I’ve been incredibly proud to represent the people of Ulster and it’s brought great joy to me personally, but also to my family and friends. I’d like to thank those closest to me for the support they have offered throughout my career.
“I’m also extremely thankful to the players, the staff and the supporters for the memories that I will carry with me for the rest of my life. I have met some amazing people along the way and we have enjoyed some amazing moments together.
Bryn Cunningham, Ulster’s Operations Director, paid tribute to Marshall:
“Paul has been an absolute pleasure to work alongside. I was still playing here when Paul first joined the squad and it’s been brilliant to see him develop into a player that would ultimately go on to represent his home province over 200 times.
“He has delivered some big performances in an Ulster jersey over the years and he can be proud of his playing career. Paul is a very popular member of the squad and his presence will definitely be missed in the seasons ahead.
“I know that Paul has the skills and personality to be a success in his career outside of rugby and I wish him and his family well.”