Smith scored with a wrist shot high to the blocker side from the left faceoff circle to make it 2-1 after stealing the puck from defenseman Victor Hedman just inside the Lightning blue line.
Smith, who had an NHL career-high 25 goals last season, scored for the second consecutive game after having one goal in the first 11 games.
"It's still early in the season, but it's obviously nice," Smith said. "It's a little bit of a confidence booster."
Jonathan Marchessault and Derek MacKenzie also scored for the Panthers (6-6-1), who began a three-game homestand.
Marchessault's goal was his seventh, matching the NHL career high he set last season when he was a member of the Lightning. He gave Florida a 1-0 lead 3:10 into the game.
MacKenzie scored into an empty net with 36 seconds left.
Ondrej Palat scored for the Lightning (7-5-1), and Ben Bishop made 29 saves.
"I thought we played a pretty good game," Bishop said. "We did everything but score. Luongo played well. It comes down to who makes the last error. Sometimes they go your way, sometimes they don't."