Terms of the deal weren't revealed, but multiple reports indicated it is worth $42.5 million.
The 5-foot-10 receiver -- a sixth-round Pittsburgh draft pick in 2010 -- hauled in 69 passes for 1,108 yards receiving last season.
The versatile threat compiled 325 punt-return yards, 737 yards returning kickoffs and 41 yards rushing, scoring a total of three touchdowns.
The Central Michigan product has made 85 grabs for 1,275 yards in 25 games over his two-year Pittsburgh career.
The signing came as the Steelers waited for wide receiver Mike Wallace to agree to a one-year franchise tender.