Terms of the deal were not been revealed, but various reports list the value at $37.5 million, for an annual average value of $7.5 million.
"I am excited we were able to reach an agreement today," Stamkos said in a statement. "This was obviously very important to me and I appreciate (team owner Jeff) Vinik and (GM) Steve Yzerman for all their work in the transformation of the franchise as we move forward. There is no place I'd rather be than as a member of the Tampa Bay Lightning."
Stamkos, 21, was second in the League last season with 45 goals, and his 91 points were fifth. He added 6 goals and 13 points -- including a trio of 3-point games -- in 18 postseason games to help the Lightning make the playoffs for the first time since 2007 and advance to the Eastern Conference Finals for the first time since 2004.
That success followed his 2009-10 season, when tied for the League lead with 51 goals and won a share of the Rocket Richard Trophy.
His 96 goals the past two seasons are the most of any player in the League.
The first pick of the 2008 Entry Draft, Stamkos has 119 goals in 243 games. He played all 82 games last season for the second straight season, and has missed just three games in his career -- all during his rookie season.
"Steven is extremely important to this franchise and is part of the foundation of our hockey team," Yzerman said. "We are very pleased to have him signed and look forward to seeing him in a Lightning uniform for years to come."