Locker spurred the Titans to 16 straight points after Tennessee returner Darius Reynaud took the game's first kickoff and kneeled down for what he thought would be a touchback.
Reynaud, however, was in the end zone, so his downing the ball counted as a safety for Pittsburgh and handed the Steelers a quick 2-0 lead.
Chris Johnson picked up 70 rushing yards on 25 carries and running back Jackie Battle scored his first touchdown for Tennessee (1-0), which pulled off an upset in its season opener.
Ben Roethlisberger was held to 21-of-33 passing for 191 yards, one touchdown and an interception as the Steelers (0-1) managed just 32 rushing yards.
The game held more bad news for Pittsburgh when it lost Pro Bowl center Maurkice Pouncey to a right knee injury.