The Burmese government has said that 85,000 people have been driven from their homes by heavy flooding.
The Irrawaddy Delta - where 130,000 people died in a cyclone in 2008 - has been worst affected.
Unusually heavy monsoon rains have inundated around 250,000 hectares of rice fields.
President Thein Sein has visited the affected areas, but damaged transport and communications networks mean the full picture is still emerging.
The government says it has set up more than 200 emergency relief centres to help those who have had to leave their homes.
Next year's rice harvest is also expected to be significantly affected. Rice is a key export for Burma and the staple diet for most of the population.
The BBC's South-East Asia correspondent Jonathan Head says the floods will test how reformed the new government in Burma is.
When Cyclone Nargis devastated communities across the Irrawaddy Delta four years ago, the then-military government arrested those who reported the scale of the disaster and refused foreign help.
This time, a civilian government which took power in 2011 and now includes opposition politicians is in charge.