November to February is the cool, dry season and the only sensible time to visit. Temperatures are pleasantly warm in the day and cool at night, the morning fog rising from the Kaladan River gives the temple ridges an unreal beauty, and the rivers are at the right level for boat travel.
March and April are very hot. May to September is the monsoon, when the river journey from Sittwe becomes slower and sometimes impossible, and many of the temple paths turn to mud. October is a transition month, often still wet but with the worst of the rains over.
Major caveat: Rakhine State has been the site of severe ethnic conflict and, since 2021, civil war. Foreign access to Mrauk-U has been restricted or impossible at various times. Check current security and permit conditions before planning a visit — and confirm directly with travel operators rather than relying on online guides.