A source told ESPN NFL Insider Adam Schefter that Clayton agreed to a one-year deal.
Clayton confirmed his return with a post to his Twitter page Wednesday.
"It's official!! Another year in the Lou! 'Grace that's all I can say,' " he wrote.
Clayton, 29, caught 10 passes in the Rams' season opener last season after St. Louis acquired him from Baltimore only days earlier.
Clayton was leading the Rams in receptions with 23 when he suffered a torn patellar tendon against Detroit in the team's fifth game. He underwent surgery and did not return.
Clayton became an unrestricted free agent this offseason. The Rams wanted him back, but only when Clayton's knee had healed sufficiently.
The Rams have some tough decisions to make at wide receiver as they prepare to meet the 53-man roster deadline Saturday. Clayton joins a group featuring Mike Sims-Walker, Brandon Gibson, Danny Amendola, Greg Salas, Austin Pettis, Mardy Gilyard, Donnie Avery and Danario Alexander.
Mike Sando covers the NFC West for ESPN.com.