Const. Jennifer Kovach, 26, was extricated from the vehicle by the Guelph fire department after the crash at 12:30 a.m. Thursday on Imperial Road South near Paisley Road, Guelph police said.
She was taken to Guelph General Hospital, where she was pronounced dead.
Kovach was alone in her cruiser when she responded to a call for assistance from another officer.
She was driving north on Imperial Road South when she "regrettably lost control of her cruiser," went over the median and collided with the bus going in the opposite direction, said Guelph police chief Brian Larkin at a Thursday afternoon news conference.
- Condolences pour in for Const. Jennifer Kovach
She suffered "significant injuries at the scene," he said.
Larkin said investigators were looking into whether road conditions were a factor in the crash. There was some snowfall at the time. But finding out exactly what happened may take some time, Larkin said.
Kovach had been on the force for four years. She was the daughter of Guelph Ward 4 Coun. Gloria Kovach, Guelph police said.
In a public statement, Guelph Mayor Karen Farbridge expressed her condolences to the Kovach family and the Guelph Police Service.
"There are no words that can adequately express our sorrow over their tragic loss," said Farbridge. "Constable Kovach will always be remembered for her sacrifice to keep our community safe."
Cam Guthrie, another councillor of Ward 4, told CBC News he had known Kovach since she was a child.
"The thing that I remember about her is the willingness to stop for the moment and she was always willing to say hi," said Guthrie. "Even when she was on the beat, she always made time."
The driver of the bus was taken to hospital suffering minor injuries and was released. There were no passengers on the bus at the time of the crash.
"This is a sad and tragic day for the Police Service and the City of Guelph. Const. Kovach exemplified the values of our service and was committed to making a difference," Guelph police said in a release.
"Jennifer was fulfilling her dream as a police officer, and we will forever remember her zest for life, her strong sense of adventure and her gregarious smile that brightened all our lives."