The area covered by the advisory extends from Windsor in the west to Kingston in the east and north to Orillia.
The ministry says hot and sunny conditions with light southerly winds are resulting in elevated smog levels across southern and parts of eastern Ontario.
The advisory will remain in effect until further notice.
A smog advisory is issued when the air quality index is expected to reach 50 — a level that can reduce lung function even in healthy people — in the next 24 hours.
Smog is a complex mixture of air pollutants that is sometimes visible as a brownish-yellow haze over cities.