The long snapper, who had been the longest-serving player at Candlestick Park, was cut ahead of yesterday's deadline for teams to reduce their active rosters to 53.
The 36-year-old said in a statement: "Today, I feel as though I've been honourably discharged from the team I love. To the various coaches that I played for, thank you. Specifically, the first staff lead by head coach Steve Mariucci, it was that group that taught me how to be a pro.
"In addition, I'd like to thank this most recent staff, led by coach Jim Harbaugh, who has returned the 49ers to playing dominate football. I love this game and these men are true football men.
"In a very real way I learned to be a man as a 49ers. Molded through the preparation and performance of game day. Completely tested while on and off the field this team, game, and group of men made me who I am today."
Jennings spent 14 seasons with the 49ers, earning two Pro Bowl call-ups.