May in Helsinki averages a daily high of 14.6°C with nights around 6.6°C. Expect roughly 9.4 wet days totalling 39.2mm. 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 May data.