September in Helsinki averages a daily high of 15.7°C with nights around 9.6°C. Expect roughly 10.9 wet days totalling 55.4mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Helsinki is June–August.
Based on HELSINKI KAISANIEMI AWS, FI (0.9km), 1995–2024. 30 years of September data.