He caught passes from rookie quarterback Trey Lance on Friday during the portion of practice open to the media. The 49ers waived wide receiver Austin Proehl on Monday to fit Lee on their 90-man roster.
Lee, 29, opted out of the 2020 season due to the COVID-19 pandemic after signing with the New England Patriots as a free agent. The Patriots released him in March.
The 2014 second-round draft choice out of USC spent the first six years of his career with the Jacksonville Jaguars, having signed a four-year contract worth $34 million in March 2018. But he played in only six games after signing that big deal, having missed the 2018 season after suffering a torn ACL, MCL and PCL in his left knee during the preseason.
Lee has 174 catches for 2,184 yards and eight touchdowns in 59 career games.