"We have intelligence from multiple sources, including our allies and our own intelligence. The evidence indicates that the plane was shot down by an Iranian surface-to-air missile," the Canadian prime minister said.
Trudeau speaking at a press conference added that "may well have been unintentional." At least 63 Canadians had died in the crash.
The Ukranian Boeing airliner with 176 onboard crashed shortly after takeoff near Tehran airport.
US President Trump said he had his suspicions about the cause of the crash, adding: "Somebody could have made a mistake."
In Britain, Prime Minister Boris Johnson said that there was a "body of information" which pointed towards a missile hitting the Ukranian airliner.
“There is now a body of information that the flight was shot down by an Iranian Surface to Air Missile. This may well have been unintentional," UK PM Johnson said in a statement.
“We are working closely with Canada and our international partners and there now needs to be a full, transparent investigation," into the plane crash, Johnson added.
Meanwhile, the US National Transportation Safety Board( NTSB) said it will join the probe into the Ukrainian Boeing airliner which went down in Iran.
The agency said it had "received formal notification" from Iran. "The NTSB has designated an accredited representative to the investigation of the crash," the agency said.
"The NTSB continues to monitor the situation surrounding the crash and evaluate its level of participation in the investigation," it said.
"As with any investigation in which the NTSB is involved, the agency will not speculate about the cause of the crash," the statement said.