On July 30, 2004, Jackson signed a five-year, $18.73 million deal with the Rams, which included a $2.5 million signing bonus. In his 2004 rookie season, he was a backup under Marshall Faulk. He made his NFL debut in the season opener against the Arizona Cardinals and had seven carries for 50 yards in the 17-10 victory. He scored his first professional touchdown on a two-yard rush against the San Francisco 49ers in Week 4. He saw an increased role toward the end of the season. In Week 13 against the 49ers, he had 26 carries for 119 yards for his first 100+-yard game. In Week 16, against the Philadelphia Eagles, he had his best performance of the season with 24 carries for 148 yards and a touchdown. As a rookie, he rushed 134 times for 673 rushing yards and four touchdowns, and had 19 receptions for 189 yards.