The couple are on a tour of Canada's Northwest with their two young children.
In Whitehorse, the capital of the Yukon territory, they dictated a message transcribed by a 90-year-old former telegraph operator that read simply: "The Duke and Duchess of Cambridge, September 2016, Whitehorse Yukon."
It was reportedly the first message sent from the century-old log Telegraph House at the MacBride Museum in more than 80 years.
"Their royal highnesses have connected Yukon's communications past with today's modern global digital technology," Yukon Tourism and Culture Minister Elaine Taylor said in a statement.
Now, visitors to the museum will be able to use the telegraph to send their own Twitter message.