Langford, a third-round draft pick out of Hampton in 2008, had played in every game in four seasons with the Miami Dolphins.
"Thanks to the @MiamiDolphins and all of the fans for the best 4yrs of my life! I'm gonna miss u guys!," Langford tweeted Saturday. "Now it's time to start a new chapter in my life here wit the St. Louis Rams and I'm pretty excited about it."
The 26-year-old Langford started 12 games in 2010, played in all 16 and was on the field for about half of the Dolphins' defensive snaps last season.
He split time at left defensive end and left defensive tackle and recorded career-highs in tackles with 47 and sacks with three. He has 7½ career sacks.
The Rams ranked 31st in rushing yards allowed and 28th in rushing yards allowed per carry. They are starting over at defensive tackle this offseason, having cut ties with Fred Robbins and letting others hit the market.
Information from ESPN.com's Mike Sando contributed to this report.