The clip showed flames erupting from the Sunwing aircraft, sending black smoke spewing into the frigid night air.
"Our plane was crashed into by another plane right after the pilot announced they were 'low on staff,'" wrote the user who posted the video, under the handle stephen_belford.
WestJet said on Twitter that the Boeing 737-800 plane had 168 guests and six crew on-board, and had arrived in Toronto via Cancun.
Waiting to proceed to the gate, the aircraft "was struck by a Sunwing aircraft pushing back from the gate," WestJet said.
"Due to the position of the aircraft on the laneway, WestJet guests required evacuation via emergency slide. Emergency crews were on hand and responded immediately," the airline tweeted.
The company did not specify if the incident resulted in injuries but said "all 168 guests and six crew are accounted for."
"We can confirm guests are safely in the terminal and they are in the process of clearing customs." In a statement posted on its social media pages Sunwing said "there were no Sunwing crew or passengers on-board at the time of the incident," adding that its aircraft had been "under tow by our ground handling service provider" prior to the collision.
Toronto's weather was clear but overnight the temperature plunged below -20 degrees Celsius (-4 degrees Fahrenheit).
The Transportation Safety Board had arrived at the scene and launched an investigation.
The collision is the second such incident at Toronto Pearson in recent months. In early August, a Canadian and a Polish passenger jet clipped wings on the ground at the airport, causing "serious" damage but no casualties.