Bettis scored on runs of 1 and 3 yards, and Chris Fuamatu-Ma'afala's 5-yard, first-quarter burst ended Pittsburgh's three-game touchdown drought against Jacksonville.
The Jaguars (2-3) fell below .500 for the first time since the 15th week of the 1996 season.
The Steelers (1-3) also played inspired defense, sacking Mark Brunell seven times and forcing two turnovers by the Jaguars' quarterback. Jacksonville was limited to two field goals by Steve Lindsey until Jamie Martin threw an 11-yard TD pass to Keenan McCardell with nine seconds left in the game.
Brunell was 15-of-32 for 137 yards with one interception and one fumble.
The Jaguars were embarrassed 43-14 by Indianapolis last Monday night and sputtered on offense for the third straight week. The defense started the game without linebacker Lonnie Marts(abdominal strain) and was further weakened when linebacker Hardy Nickerson pulled his right hamstring in the second quarter.
The Steelers led 17-6 at the half despite making several mistakes that might have hurt them in past meetings with the Jaguars in the AFC Central rivalry. Jacksonville, however, was only able to score three points off a blocked punt, an interception and a kickoff that sailed out of bounds.
Brunell, questionable to play early in the week after straining his left calf against the Colts, didn't complete a pass until a minute into the second quarter. The Jaguars were held to minus 4 yards in the opening quarter and failed to get into the end zone after Donovin Darius blocked Josh Miller's punt and Greg DeLong recovered at the Steelers 4.
Pittsburgh drove 53 yards and took a 7-3 lead on Fuamatu-Ma'afala's touchdown, a drive that was helped by a personal foul penalty on Gary Walker for a late hit on Stewart.
Stewart scrambled for 34 yards and completed passes of 15 yards to Hines Ward and 14 yards to Plaxico Burress to set up a second-quarter field goal. He then took advantage of Lee Flowers' interception to put the Steelers up 17-3.
Lindsey kicked a 48-yard field goal for Jacksonville just before the half, but the Steelers kept the Jaguars from getting back in the game by stopping Brunell on downs twice inside the Pittsburgh 20.
McCardell finished with 11 receptions for 137 yards.
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