"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," 49ers general manager John Lynch said in a statement. "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."
Marsh, a fourth-round Seattle Seahawks draft pick in 2014, posted 10 tackles, two sacks and two forced fumbles in his time as a 49er this past season. The DL has six career sacks in his four seasons in the NFL.