He made his professional regular season debut and first career start during the Green Bay Packers' season-opener against the Minnesota Vikings and made five combined tackles (one solo) during their 30–25 loss. On September 14, 2003, Barnett collected a season-high 14 combined tackles (12 solo), deflected one pass, and made his first career interception during a 31–6 victory against the Detroit Lions in Week 2. Barnett intercepted a pass by Lions' quarterback Joey Harrington, that was originally intended for tight end Mikhael Ricks, and returned it for a 13-yard gain during the fourth quarter. In Week 4, he made four combined tackles (three solo) and made his first career sack on Bears' quarterback Kordell Stewart for a five-yard loss during the first quarter of a 38–23 victory at the Chicago Bears. Barnett was inactive for the Packers' Week 13 loss at the Detroit Lions after sustaining an ankle injury during a 20–10 win against the San Francisco 49ers the previous week. He finished his rookie season in 2003 with 112 combined tackle (86 solo), three pass deflections, three interceptions, and two sacks in 15 games and 15 starts.