Rossum signed with the San Francisco 49ers on February 29, 2008. The 49ers used him primarily as a kick returner, although he occasionally played as a receiver during the season, scoring a rushing touchdown on a reverse against the Detroit Lions – the first rushing touchdown of his career. Rossum became the first 49ers defensive back to score an offensive touchdown since Dicky Moegle in 1955 (December 11, 1955, vs. Baltimore Colts). He returned 15 punts for 223 yards, and returned 47 kickoffs for 1,259 yards and a touchdown (a 104-yard return, the longest of his NFL career, against the Arizona Cardinals on a Monday Night Football game). Rossum finished sixth in the NFL and third in the NFC with a 26.8 average on kickoff returns and a 14.9 yard average on punt returns. Due to his strong performance, he was named a second alternate returner for the 2009 Pro Bowl.