The world's longest international frontier at 8,900 kilometers (5,500 miles) was closed to travellers on March 21, but trade in goods has continued.
Trudeau at his daily briefing said this second extension of the border closure was "important ... to keep people in both of our countries safe."
The prime minister also left open the possibility of further extensions, saying: "We will continue to watch carefully what's happening elsewhere in the world and around us as we make decisions on next steps."
Some Can$2.4 billion (US$1.7 billion) worth of goods and more than 400,000 people crossed the border each day on average, prior to the pandemic.
Since its closing in March, the number of travellers crossing from one country to the other has fallen to a trickle.