Ottawa Weather in September
In September, Ottawa averages a high of 21°C (69°F) and a low of 10°C (50°F), from 54 years of daily records (1971–2026).
21°C
Average high
69°F
10°C
Average low
50°F
35°C
Record high
96°F · September 9, 2002
-3°C
Record low
27°F · September 29, 2000
What September Is Like in Ottawa
- September in Ottawa has become 1.3°C (2.4°F) warmer on average: the 2011–2025 average against 1970–1989.
- The warmest September in the record was 2015; the coldest was 1995.
At least 1 mm of rain falls on 32% of September days, and the month averages 84 mm in total.
Warmest: 2002 (24.4°C)Latest: 2025 (22.9°C)10-year averageLong-run average (20.5°C)
Look Up a Day in September
See the weather in Ottawa on any September date, year by year.
Source: NOAA GHCN-Daily, Ottawa Cda (1971–2023); Ottawa Cda Rcs (2000–2026). Averages cover 1971–2026. How we compute this