A league source told ESPN.com's Pierre LeBrun that the deal averages $4.1 million per season.
Hjalmarsson was set to become an unrestricted free agent after the 2013-14 season. His new contract will run through the 2018-19 season.
"It's been a pretty unbelievable summer, that's for sure," Hjalmarsson said Wednesday. "It's going to be tough to top this one. It's been a lot of celebrating, a lot of fun stuff going on. But I'm really excited with the extension here, and I'm really looking forward to next season and that me and my wife get to stay in Chicago for six more years.
"As I've been saying before, I love the city, I love the people in Chicago. They're really friendly. Playing in the United Center in front of 23,000 people every game, it's really inspiring. On top of that, we have a good team that can compete for the Cup every year. I really wanted to stay in Chicago, and I'm really glad I got the opportunity to do that."
Selected in the fourth round of the 2005 draft, the 26-year-old Hjalmarsson has nine goals, 47 assists and a plus-48 rating in 306 regular-season games over six seasons. He was a key contributor during the Hawks' 2010 and 2013 Stanley Cup championships.
Hjalmarsson had two goals, eight assists and a career-high plus-15 rating in 46 regular-season games last season. He also had five assists and a plus-10 rating in 23 playoffs games.
Hjalmarsson's deal is the second long-term extension the Blackhawks have announced this week. The team and goaltender Corey Crawford agreed to a six-year contract extension Monday.
"I think part of the plan is determining which of your players are key and critical to the success of your team and making sure you have them for a long time," Blackhawks general manager Stan Bowman said. "Look at the role Niklas has played on our team, and he's a warrior the way he plays. He's a heart-and-soul guy, very relied upon by our coaching staff. They trust him.
"He does a lot of things other guys don't want to do -- penalty killing, blocking shots, defending, keeping the puck out of the net. That's really important when we got a lot of guys who play different styles than that. I think he's a great complement. When you have a player who hasn't really reached his best years yet, I think he's just coming into his prime at this point, you want to keep someone like that. He's been improving each year, and I expect him to continue to do so. I think for that reason it's an easy decision to commit to him because he's a warrior."