May in Rapid City averages a daily high of 18.3°C with nights around 7°C. Expect roughly 14.1 wet days totalling 101.2mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Rapid City is September.
Based on RAPID CITY WEATHER FORECAST OFFICE, SD US (1.8km), 1995–2024. 28 years of May data.