September in Belfast averages a daily high of 17.2°C with nights around 10.3°C. Expect roughly 14.4 wet days totalling 77mm. Generally workable, with some weather to plan around.
Across the year, the best time to visit Belfast is June–September.
Based on STORMONT CASTLE, UK (6.3km), 1995–2024. 28 years of September data.