Rams coach Jeff Fisher said the move was needed to clear space to help at a position other than wide receiver.
"We appreciate Austin's contributions, everything that he's done here," Fisher said. "He made a lot of plays for us. We needed to make some moves with respect to some other positions. I'm not going to go into detail about what we're going to do but we needed a roster spot."
However, a league source said Monday afternoon that Pettis and practice squad receiver Emory Blake were late for a team activity over the weekend. The Rams also released Blake on Monday.
That doesn't mean there's no validity to the reason Fisher gave but it surely played a part in Pettis' release and was likely the major factor in cutting Blake. Pettis also was a pregame inactive for the Rams on Sunday afternoon, clearing the way for Chris Givens to return to action after sitting out the previous two weeks.
Givens made a key 30-yard catch to set up the Rams' final touchdown in the fourth quarter.
"Nobody likes being inactive, especially if you've been a contributor like he has been," Fisher said. "He worked right through it. Some guys can mope around and not work hard, and he worked even harder, which was good to see."
Pettis was already in something of a precarious roster spot after he restructured his contract in early September, agreeing to take a pay cut that saved the Rams $481,000 against the salary cap. The former third-round draft choice out of Boise State had 12 catches for 118 yards and a touchdown in five games this season. In three-plus seasons with the Rams, Pettis had 107 catches for 1,034 yards and nine touchdowns.