Moorea has the standard tropical Polynesian climate — warm all year, with two seasons defined by rainfall rather than temperature.
May to October is the dry season and the obvious time to visit: less rain, lower humidity, daytime temperatures around 26–28°C and reliable trade winds that keep things comfortable. This is also the peak season for whales — humpback mothers and calves congregate in Moorea's sheltered lagoon from July to October to give birth and rest before the migration south, and licensed operators offer in-water encounters.
November to April is the wet season: warmer (around 30°C), more humid, and prone to short, heavy daily downpours, especially in January and February. Tropical cyclones are rare but possible. The lagoon water is at its warmest. This is also the off-peak season for tourism, meaning lower prices and emptier beaches.
The Heiva i Tahiti festival in July spills over to Moorea with traditional dance, drumming and outrigger-canoe racing — worth timing a visit around if you have the option.