NFL Network's Ian Rapoport reports that Britt's deal is for two years and worth up to almost $14 million, with $4.3 million guaranteed.
"It feels awesome," Britt said. "I'm going to paint my horns back on. We've got great fans, great coaches, great people around us, and I'm just excited for the 2015 season."
The 26-year-old caught 48 passes last season for 748 yards, his highest total since 2010. He also added three touchdowns and made 13 starts.
Britt played the first five years of his career with the Tennessee Titans, who drafted him in the first round in 2009.
For his career, Britt has 3,198 receiving yards and 22 touchdown catches.
St. Louis went 6-10 last season, finishing in fourth in the NFC West.
- Molly Geary