El Mahdi Jamali, 18, and Sabrine Djermane, 19, will remain in detention until the end of the legal case against them.
Family and friends were present in the courtroom as the decision came down.
The pair, a couple, have been detained since their arrests in mid-April.
From the outset, Sansfacon said granting bail would have been "particularly difficult" given the circumstances.
A bail hearing was held over four days earlier this month, but there is a publication ban on the evidence.
They each face four charges: attempting to leave Canada to commit a terror act abroad; possession of an explosive substance; facilitating a terrorist act; and committing an act under the direction or for the profit of a terrorist organization.
Jamali and Djermane, both students at College de Maisonneuve, a Montreal junior college, were arrested in April as what the Crown called a "preventive measure."
Prosecutors wanted them to sign peace bonds that would set out conditions and restrictions on their day-to-day lives.
But several days after their arrest, they reappeared before a judge to face the criminal charges.
Crown prosecutor Lyne Decarie had opposed bail.
The case returns to court on July 13.