The team announced the contract on Wednesday. Blackhawks general manager Stan Bowman says he believes Leddy will be essential to the team's long-term success.
Leddy played in every game this season for Chicago, helping the Blackhawks win the Stanley Cup. He had six goals and 12 assists in the regular season, and then had two assists in 23 playoff games.
Leddy was selected by Minnesota in the first round of the 2009 draft. He was traded to Chicago in February 2010.
The 22-year-old Leddy, who was set to become a restricted free agent, has 13 goals and 49 assists in 176 regular-season games over three seasons.