The government says that means no indoor organized public events or social gatherings except with members of the same household.
The lockdown will limit non-essential retailers to curbside pickup, ban indoor dining at restaurants, close personal care services, and shutter indoor sports facilities.
Grocery stores, pharmacies, and big box retailers will be deemed essential and remain open, with capacity limits.
Schools and child care centres will remain open.
The lockdown will last a minimum of 28 days and the province says it will fine people $750 for violating public health rules.
Premier Doug Ford says the lockdown is needed to get alarmingly high levels of community transmission in Toronto and Peel under control.
The measures come as Ontario reports 1,418 new cases of COVID-19 today, along with eight new deaths related to the virus.
The government said there were 400 new cases in Peel Region, 393 in Toronto and 168 in York Region.
The latest figures have pushed the province over the 100,000 case mark, for a total of 100,790 infections.