Much of the country has been hit by flooding and landslides over the last few weeks.
The government's Relief and Resettlement Department estimates that 110,000 people have been severely affected by the floods, particularly in the Sagaing region and Kachin and Shan states, accounting for 21 deaths in July.
On Saturday, local officials reported nine more deaths, most of them from a boat that capsized in Kachin. They said seven townships had been devastated by flooding.
President Thein Sein visited Kalay township in Sagaing, one of the four regions that were declared disaster zones on Friday.
Thein Sein had been coming under criticism, especially on social media, for not doing more to deal with the emergency.
On Thursday, he urged Cabinet ministers to go out into the field to supervise flood relief operations, saying that since July 16, some areas of the country had become inundated by heavy rains that destroyed farmland, roads, rail lines, bridges and houses. Roads from central to northern Myanmar have been especially affected.
An Information Ministry announcement late Friday said Thein Sen had issued an emergency declaration covering the Magway and Sagaing regions and Chin and Rakhine states, some of the hardest-hit areas. The announcement did not specify what practical measures would be taken.
There is particular concern about Rakhine, where a tropical storm was moving from neighboring Bangladesh. The situation in Rakhine is considered especially dangerous, because more than 100,000 internally displaced people who fled their homes due to civil conflict in recent years live in poorly built and badly situated camps.