The 25-year old comes to the Reds in exchange for a third-round draft choice in the 2011 MLS SuperDraft.
Toronto coach Preki Radosavljevic announced the deal during the week and was delighted to have the former LA Galaxy man at his disposal.
"We are excited to have him on board with us," Preki said.
"He's a good quality, young defender, and there are many things I like about him. He showed a lot of promise with the (Los Angeles) Galaxy and we hope he can add some toughness to our back line."
Harden was a second-round pick for the Galaxy in the 2007 draft and played 24 matches in his debut MLS season but in early 2008 he decided to take some time out from the game and return to university.
He went back to the University of Washington before teaching at a children's centre in the slums of Nairobi, Kenya but is glad to be back playing professionally and after a stint with the Rapids is looking forward to repaying the faith sown in him by Toronto.
"For central defenders it is important to have strengths as a ball winner and be aggressive and communicate really well," Harden told TFC-TV in Bradenton. "But ... I keep the ball, keep possession and play out of the back and I think that is one of the reasons why Preki likes me as a player."