The death toll at the Newton school was 26, plus the gunman, and an adult was found dead at a second crime scene, police said.
Prime Minister Stephen Harper reacted on Twitter, just minutes after finding out about the massacre.
"The news is just awful. The thoughts and prayers of Canadians are with the students and families in Connecticut affected by this senseless violence," he wrote.
Foreign Affairs Minister John Baird did the same, saying: "Deeply sad over the sickening news out of Newtown. Our thoughts are with our American friends during this most difficult time."
In a statement, U.S. Ambassador to Canada David Jacobson said that words couldn't express his sorrow.
"Our thoughts and prayers are with the families who have been touched by this horror," he said. "I want to thank the many Canadians who have shared with me, and with all Americans, their heartfelt thoughts and condolences."
Calling the shooting "senseless," Opposition Leader Thomas Mulcair sent his "condolences to the families of the victims — and in hugging our own children a little tighter."
"Schools are meant to be safe environments where children learn, grow and are nurtured. To see so many children targeted like this is an affront to everything we hold dear," Mulcair said.
Echoing the NDP leader's comments, interim Liberal Leader Bob Rae said that "it is said that 'sometimes the world just breaks your heart.' This is one of those moments. This senseless act of violence leaves us numb."