He made his professional regular season debut in the New England Patriots' season-opener against the Buffalo Bills and recorded one tackle in their 25–24 victory. Chung made his first career tackle with teammate Darius Butler on cornerback Leodis McKelvin during a 21-yard kick return in the fourth quarter. On October 16, 2009, Chung collected a season-high eight combined tackles, deflected a pass, and made his first career interception off a pass by quarterback Vince Young as the Patriots routed the Tennessee Titans 59–0 in Week 6. The following week, he collected four solo tackles and made his first career sack on quarterback Josh Johnson in the Patriots' 35–7 in over the Tampa Bay Buccaneers at Wembley Stadium in Week 7. In Week 10, Chung earned his first career start and recorded five combined tackles during a 35–34 loss at the Indianapolis Colts. He finished his rookie season with 37 combined tackles (25 solo), two sacks, a pass deflection, and an interception in 16 games and one start.