Nearly half a dozen districts in Seemanchal and Koshi areas in the state were the worst affected following heavy rains during the past three days.
All major rivers in Bihar are in spate after heavy rains in their respective catchment areas in Nepal and Bihar.
Floodwaters have submerged hundreds of villages, affecting thousands of people. So far, no casualty has been reported due to Bihar floods, officials said.
The state government has placed its staff on alert, ordering the evacuation of residents from flood-affected Kishanganj, Araria, Purnea, Katihar, West Champaran, Saharsa, and Supaul districts, Disaster Management Department officials said.