Planning for the weather is a good way to make your trip to Hanoi relaxing and enjoyable, especially when you’re splurging. The hottest months are usually June and July with an average temp of 85°F, while the coldest months are January and December with an average of 67°F. The rainiest months in Hanoi are August, July, September, and May, with each month seeing an average of 11 inches of rainfall.