September in Ottawa averages a daily high of 21.7°C with nights around 10.8°C. Expect roughly 10.5 wet days totalling 87.1mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Ottawa is May–September.
Based on OTTAWA CDA, ON CA (3.4km), 1995–2024. 29 years of September data.