On July 29, 2011, the Tampa Bay Buccaneers signed Foster to a four-year, $2.78 million contract that includes a signing bonus of $525,000.

Throughout training camp, Foster competed to be the starting middle linebacker against Tyrone McKenzie.
His development and conversion to middle linebacker was delayed due to the 2011 NFL lockout.
On August 24, 2011, Foster was fined $20,000 for a hit  he delivered to Chad Ochocinco during the Buccaneers' 31–14 loss to the New England Patriots in their second preseason game.
Ochocinco offered to pay the fine for him, as he disagreed with the league's decision, but the league stated that he would not be able to.
Head coach Raheem Morris named Foster the starting middle linebacker to begin the regular season after he had an impressive showing during the preseason.

He made his professional regular season debut in the Tampa Bay Buccaneers' season-opener against the Detroit Lions and recorded seven combined tackles in their 27–20 loss.
On September 18, 2011, Foster made his first career start and collected a season-high ten combined tackles (seven solo), forced a fumble, and made his first career sack during a 24–20 victory at the Minnesota Vikings in Week 2.
Foster made his first career sack on Vikings' quarterback Donovan McNabb for a six-yard loss in the second quarter.
In Week 14, he recorded six combined tackles, deflected a pass, and made his first career interception in the Buccaneers' 41–14 loss at the Jacksonville Jaguars.
Foster made his first career interception off a pass by Jaguars' quarterback Blaine Gabbert, that was originally intended for tight end Marcedes Lewis, in the endzone to secure a touchback during the third quarter.
He finished his rookie season in 2011 with 84 combined tackles (58 solo), two pass deflections, two sacks, a forced fumble, and an interception in 16 games and 15 starts.
On July 29, 2011, the Tampa Bay Buccaneers signed Foster to a four-year, $2.78 million contract that includes a signing bonus of $525,000.
Throughout training camp, Foster competed to be the starting middle linebacker against Tyrone McKenzie. His development and conversion to middle linebacker was delayed due to the 2011 NFL lockout. On August 24, 2011, Foster was fined $20,000 for a hit  he delivered to Chad Ochocinco during the Buccaneers' 31–14 loss to the New England Patriots in their second preseason game. Ochocinco offered to pay the fine for him, as he disagreed with the league's decision, but the league stated that he would not be able to. Head coach Raheem Morris named Foster the starting middle linebacker to begin the regular season after he had an impressive showing during the preseason.
He made his professional regular season debut in the Tampa Bay Buccaneers' season-opener against the Detroit Lions and recorded seven combined tackles in their 27–20 loss. On September 18, 2011, Foster made his first career start and collected a season-high ten combined tackles (seven solo), forced a fumble, and made his first career sack during a 24–20 victory at the Minnesota Vikings in Week 2. Foster made his first career sack on Vikings' quarterback Donovan McNabb for a six-yard loss in the second quarter. In Week 14, he recorded six combined tackles, deflected a pass, and made his first career interception in the Buccaneers' 41–14 loss at the Jacksonville Jaguars. Foster made his first career interception off a pass by Jaguars' quarterback Blaine Gabbert, that was originally intended for tight end Marcedes Lewis, in the endzone to secure a touchback during the third quarter. He finished his rookie season in 2011 with 84 combined tackles (58 solo), two pass deflections, two sacks, a forced fumble, and an interception in 16 games and 15 starts.
Mason Foster