Hawaii weather in September offers pleasant temperatures between 73 and 89 degrees Fahrenheit. Trade winds from the northeast somewhat calm the high humidity. The rainy season doesn't begin until November, so September rain tends to be mild-to-moderate with a higher chance on the northernmost islands.

September is widely considered one of the best "hidden gem" months to visit Hawaii. The weather remains hot and "summery," with average daytime temperatures in the mid-to-high 80s (30°C+) and very little rainfall, as the wet season doesn't typically begin until November. Most importantly, September has the warmest ocean temperatures of the entire year, making it perfect for snorkeling and swimming. From a crowd perspective, September sees a massive drop in tourism once the Labor Day holiday concludes and children return to school. This "shoulder season" status means you will find significantly cheaper hotel rates—often 20% to 30% lower than in July or December—and much better availability for popular activities like the Road to Hana or Pearl Harbor tours. One minor caveat is that September is technically the peak of hurricane season in the Central Pacific; while direct hits are extremely rare, you may occasionally experience "muggy" days with higher humidity if a tropical system is passing nearby.