Lake Ontario water temperature
Lake Ontario, NW Lake Ontario Ajax · Lake
The current water temperature at Lake Ontario is 18.0 °C (as of September 20, 2026, source: NOAA NDBC).
Lakes stratify in summer: bathing-warm on top, markedly colder a few metres down. Measurements are taken at the surface — right where you swim.
Measurement series & statistics
This measurement series includes 998 data points since July 22, 2026.
Lowest recorded value: 11.1 °C on July 23, 2026 · highest value: 24.5 °C on August 7, 2026.
In September, temperatures have ranged between 15.8 °C and 22.4 °C.
The current value is 2.1 °C below the September average of this series.
Nearby waters
More measuring points: Lake Ontario
Frequently asked questions
Can you swim at Lake Ontario right now?
The current water temperature is 18.0 °C. See live swimming conditions, weather and quality in the BrrChecker app.
How warm is the water at Lake Ontario?
Currently 18.0 °C (updated continuously).
How warm is at Lake Ontario in September?
In September, the water temperature recently averaged 20.1 °C.
When is the water warmest at Lake Ontario?
Across the measurement record, August is on average the warmest month (20.2 °C).
