The 35-year-old defenceman signed a ceremonial contract with the Flames on Monday to make it official.
Regehr played 11 of his 15 NHL seasons in Calgary before he was traded to the Buffalo Sabres in 2011 and then dealt to the Kings.
He won a Stanley Cup with Los Angeles in 2014, and made the following 2014-15 season his last.
"I always felt that I was a Calgary Flame and when I look back on my career, that's the team that I identify the most with, having spent the most time here, having a big group of family and friends and a support network here in Calgary," Regehr said Monday at a news conference.
"I put a lot of time and effort into the team when I was here. It's a nice way to celebrate that."
The six-foot-three, 225-pound defender had 36 goals and 163 assists in 1,090 NHL games.
The punishing shut-down defenceman was an integral part of Calgary's run to the 2004 Stanley Cup final, which the Flames lost in seven game to the Tampa Bay Lightning.
Regehr ranks second all-time in games played for the Flames with 826. He was an assistant captain of the team for seven seasons.
"No matter where he plays, no matter what he does, he's always going to be a Calgary Flame," said Flames assistant general manager Craig Conroy, who was also Regehr's teammate.
Regehr was born in Brazil, but grew up in Rosthern, Sask. He was a first-round draft pick of the Colorado Avalanche, who traded his rights to the Flames in a package deal that included Theoren Fleury going to the Avs.
Regehr's career was nearly derailed by a serious car accident at the age of 19. He made his NHL debut less than four months after breaking both legs in the accident.
"Going through that situation and how emotional that was and how tough physically it was and all the rehab I had to go through as a 19-year-old, I felt different," Regehr recalled. "I wanted to make the most of any opportunity that I got."
Regehr played for Canada in the 2000 and 2005 world championships, as well as the 2006 Winter Olympics.
Regehr, his wife and two sons now make Calgary their home.
The combination of winning a Stanley Cup in front of family and friends, as well as his body wearing down, prompted Regehr to retire.
"The body knew it was time and also not needing to chase a championship around anymore, there were things that fell into place that kind of made sense," he said.