The "Super Tuesday" strike involved eight school districts, including Toronto, the Toronto Star reported. In Toronto, 20,000 teachers were on picket lines instead of in classrooms.
Classes were canceled for 400,000 students.
The Elementary Teachers' Association of Ontario and other teacher unions are using the one-day strikes in an attempt to force collective bargaining. Earlier this year, the provincial assembly adopted Bill 115, which allows the Ontario government to impose a contract if none has been negotiated by Dec. 31.
Steph Dawson, a librarian and teacher, was outside an elementary school in Toronto.
"It's a way to bring us all together and show we support democracy," she said. "This isn't about our wage cuts, this isn't about our sick days, it's about the government taking away our democratic rights."
Education Minister Laurel Broten has said she would intervene to stop the strikes only if they keep going for more than a day. More one-day strikes are expected through Thursday.