The five-year deal is worth $17 million and includes a signing bonus believed to be about $4 million.
A versatile lineman who started 36 games during his four seasons in Tampa Bay, Mahan was arguably one of the top offensive linemen still available on the free agent market. He will be reunited with Pittsburgh rookie coach Mike Tomlin, a former assistant on the Bucs' staff.
Mahan also drew interest from Seattle and Cleveland in the past week, and Tampa Bay attempted to re-sign him as well.
Exactly where Mahan fits in on a Pittsburgh offensive line that will undergo some changes in 2007 will bear watching. The former Notre Dame standout started at center and both guard spots at various times during his Bucs tenure.
Mahan started eight games at center in 2004, all 16 games at right guard in 2005 and 12 contests at left guard in 2006.
The most likely scenario is that Mahan will compete with holders Chukky Okobi and Marvin Philip for the starting job at center, a position vacated by the retirement of former Pro Bowler Jeff Hartings last month. Okobi, a six-year veteran, had been groomed to take over the center spot, but it is believed that the new coaching staff doesn't regard him as highly as the former one did.
There has also been speculation that the Steelers will consider moving starting right guard Kendall Simmons to center, a position he has played sparingly in the past. If that switch is made, Mahan could end up playing guard.
A fifth-round choice of the Bucs in the 2003 draft, Mahan has appeared in 57 regular-season games.
Len Pasquarelli is a senior NFL writer for ESPN.com.