Marsh's two-year deal has a base value of $7.7 million with $3.1 million guaranteed. The maximum value of the deal, with incentives, is reportedly $12.7 million.
"Since joining the team late last season, Cassius has impressed us with his unique skill set and the tremendous energy and passion he brings to all phases of the game," said 49ers general manager John Lynch via a statement issued by the team on Wednesday. "Cassius is a great example of our desire to reward our own players with contract extensions as we continue to build out our roster. We're excited to have him as a 49er moving forward."
The 49ers initially claimed Marsh off of waivers from the New England Patriots on November 22, 2017. He appeared in each of San Francisco's final six games of the season. During that time, he registered 11 tackles, two sacks, and two forced fumbles.
Before being traded to New England on September 2, 2017, Marsh spent three seasons with the Seattle Seahawks – the team that drafted him in the fourth round in 2014.
In four NFL seasons, Marsh has compiled 85 combined tackles, six sacks, and four forced fumbles.