The 21-year-old, a first-overall pick by the Lightning in the 2008 NHL draft, has racked up 119 goals and 113 assists for 232 points over three seasons, including 45 goals and 46 assists in 82 games this past season.
"Steven is extremely important to this franchise and is part of the foundation of our hockey team," Tampa Bay General Manager Steve Yzerman said. "We are very pleased to have him signed and look forward to seeing him in a Lightning uniform for years to come."
Stamkos contributed six goals and 13 points in 18 contests in his first post-season appearance last season as the Lightning fell one victory short of the Stanley Cup Finals.
The deal wraps up a franchise-type player for the Bolts after Stamkos was rumored to have been courted by several teams, including Toronto and Philadelphia.