The 49ers claimed defensive lineman Cassius Marsh off waivers from the Patriots, Field Yates of ESPN reports.
The Patriots sent 2018 fifth- and seventh-round picks to the Seahawks for Marsh. He made 19 tackles, a sack and a forced fumble in nine games with New England before they cut him.
Marsh, a fourth-round pick of the Seahawks in 2014, played in 37 games in Seattle before the trade.
The 49ers claimed defensive lineman Sheldon Day off waivers from the Jaguars earlier this week.