Timonen, who will turn 40 on March 18, was diagnosed with blood clots in his leg and lungs in August. He's been skating on his own Feb. 6 and had been practicing with the Flyers since Feb. 17.
"My only goal is to win the [Stanley] Cup," Timonen said after the trade. "This isn't about money or anything else. I had a good seven years with the Flyers; I always thought I would retire here but this is the right move."
Philadelphia receives a second-round pick in the 2015 NHL Draft and a conditional fourth-round pick in the 2016 draft. If the Blackhawks win two playoff rounds and Timonen plays in at least 50 percent of the games, the fourth-round pick becomes a third-round pick in 2016.
"We've been working on this one for a while," Blackhawks general manager Stan Bowman said. "He was the defenseman we really wanted. He's been a high-level defenseman for a long time."