Pettis was picked in the third round of the 2011 NFL draft out of Boise State and was in his fourth season in St. Louis. He played in the Rams' first five games this season, then was inactive against the Seahawks last Sunday.
FOX Sports' Mike Garafolo reports that Pettis was late for a team meeting on Saturday.
The receiver had 12 catches for 118 yards and a touchdown in 2014. Last year, Pettis had 399 yards and four touchdowns on 38 catches. He has nine career touchdowns.
Pettis has also served as a punt and kick returner for the Rams, but returned just three punts last season and this season.
The 26-year-old is now a free agent.
St. Louis (2-4) plays the Kansas City Chiefs (3-3) on Sunday.
- Molly Geary