The 34-year-old Scot had not played since March, sidelined by recurring leg injuries.
Caldwell's teammates and employer paid tribute to their former captain for his professionalism, leadership and counsel.
"A massive role model for us," said young winger Daniel Lovitz
Said goalkeeper Joe Bendik: "Individually he made a big impact on my life as a person and also as a player ... He's an excellent teammate and an excellent friend."
"A proper man," said general manager Tim Bezbatchenko.
"A true leader," added coach Greg Vanney.
Caldwell is staying in the fold. His new role is that of director of corporate development with Maple Leaf Sports & Entertainment, the parent company of Toronto FC.
New job aside, it was clearly a day Caldwell had hoped not to see for several years.
"Obviously it's very difficult and sad, retiring and stopping playing the game that I love," Caldwell told reporters. "But also I'm very excited about my new role with MLSE ... and proud and privileged to be lucky enough to have played for so many magnificent clubs in some of the best stadiums in the world."
Caldwell's 17-year career began in 1998 at Newcastle United. He also played for Blackpool, Bradford City, Leeds United, Sunderland, Burnley, Wigan and Birmingham City.
Caldwell won 12 caps for his country, last turning out for Scotland in February 2011.
He arrived in Toronto on loan from Birmingham, making his MLS debut against Columbus in May 2013. His final appearance was March 14, also against Columbus, when he was substituted at halftime due to injury.
Forty of his 49 appearances for TFC were as skipper. He gave up the armband to U.S. international Michael Bradley this season but still leaves as the franchise's second longest-serving captain behind Jim Brennan.
Caldwell brought defensive stability and accountability to Toronto, not to mention a sharp fashion sense. Suspenders and argyle socks were only part of his clothing arsenal.
The club was never able to find him a consistent partner at centre back, however. It was hoped that would change this season when Toronto brought in Polish international Damien Perquis but Caldwell couldn't get healthy.
In coming to Canada, Caldwell joined a struggling franchise. But he saw potential.
"The most important thing for me is that I feel it's a football club and a franchise that is not going to stop until it gets the best people, the best players and the right players for success," he said in July 2013.
"That's why I came here. I'm not here because it's a beautiful city and it's a holiday, I'm here to win. First and foremost that's to get into playoffs and then it's to win the whole thing. If I go home and I haven't achieved that, It'll be a big disappointment for me."
Today Toronto sits third in the East at 7-7-3 — that's one more win than 2013 when the club went 6-17-11 — and is headed to the playoffs for the first time with designed players Jozy Altidore, Sebastian Giovinco and Bradley leading the way.
In retiring, Caldwell opens up cap space and an international spot for Toronto. His salary was listed as US$364,165 last season but he likely made more with allocation money reducing the load.
Caldwell took the job as captain seriously and was always front and centre in the dressing room, win or lose.
Toronto, however, elected to give Bradley the keys to the club for the 2015 season.
The transition was bungled with MSLE CEO Tim Leiweke saying Bradley had been involved in player personnel talks during the off-season. Asked if he was involved in similar discussions, Caldwell said no.
"I love being captain of this football club,'' he said on Jan. 26. "I've captained every team I've played for since I was 15 so it's something that I relish, I enjoy and I think I'm quite good at it. It's a great honour to be captain of any club. I don't treat it lightly. I love being captain of Toronto. So I hope to stay that.''
On Feb. 10, Toronto announced Bradley was captain.
Caldwell never complained publicly. And while Bradley showed Caldwell nothing but respect in taking over, the veteran defender deserved better.
Now he is part of the corporate braintrust, working under chief commercial officer Dave Hopkinson.
"Finally to TFC, I appreciate and love this club more than all the others," Caldwell said in a letter to fans. "The opportunity to stay here and be a part of the success that will come is a privilege."
Classy to the end.
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