September in Seattle averages a daily high of 21.7°C with nights around 12.4°C. Expect roughly 8 wet days totalling 36mm. Comfortable conditions for being outdoors all day.
Across the year, the best time to visit Seattle is July–August.
Based on SEATTLE BOEING FIELD, WA US (6.9km), 1995–2024. 26 years of September data.