Travelling with his wife Queen Camilla, the king met on the airport tarmac on Monday with Prime Minister Mark Carney and Governor General Mary Simon, his representative in Canada.
Following an invitation from Carney, Charles will open parliament on Tuesday, the first time a British monarch has carried out the duty since his mother, the late Queen Elizabeth, did so 68 years ago.
The 76-year-old king is undergoing cancer treatment, which has limited his workload, so the two-day trip shows his commitment to Canada, one of 15 countries where he is monarch.
Trump has repeatedly expressed a desire to annex Canada, a proposition fiercely rebuffed by Carney, whose election win last month came partly on the back of that stance.
"The prime minister has made it clear that Canada is not for sale now, is not for sale ever," Canada's envoy to the UK, Ralph Goodale, told reporters during a visit last week by Charles to Canada's high commission.
"The king, as head of state, will reinforce the power and the strength of that message."
Charles has made subtle signals of his backing for Canada in recent months, wearing Canadian medals, calling himself the king of Canada, and describing its flag as "a symbol that never fails to elicit a sense of pride and admiration."
However, he faces a tricky balancing act as British Prime Minister Keir Starmer is working to keep Trump onside over Ukraine and trade ties.
When Starmer visited the White House in February he made great show of an invitation from Charles for an unprecedented second state visit for Trump, whose mother was born in Britain and who has repeatedly praised the British royal family.
Carney, the former governor of the Bank of England, said that gesture had irked Canadians.
Charles' trip will be his first visit to the former British colony since becoming king in September 2022.
Later, the royal couple visited a large park in Ottawa and met vendors and artists. The king will also participate in a ceremonial puck drop to launch a street hockey demonstration before planting a tree in another part of the city.
It will be just the third time a sovereign has delivered a "speech from the throne" in the Canadian Senate, an event that opens every new session of parliament.
"This historic honour matches the weight of our times. It speaks to our enduring tradition and friendship, to the vitality of our constitutional monarchy and our distinct identity, and to the historic ties that crises only fortify," the Prime Minister's Office said in a statement.
Charles and Camilla will travel to the Senate in a ceremonial horse-drawn carriage with a 28-horse escort to deliver a 25-minute speech written by Carney's government.
