The 28-year-old netminder posted a 32-22-4 record last season with four shutouts in 59 games. Lehtonen recorded a .922 save percentage and 2.33 goals-against average.
"We believe that his best days as a goaltender are ahead and that his work ethic and leadership will help guide this team into a perennial Stanley Cup contender."
Lehtonen will be paid $5.9 million per year through the 2017-'18 season.
In eight NHL seasons with the Stars and Atlanta Thrashers, the six-foot-four, 217-pound Helsinki native has a 166-133-32 record with a 2.71 goals-against average and 21 shutouts.