He has also urged his fans to help the victims.
A cheque of Rs. 25 lakh was sent by 'Dangal' star through courier and was received at the office of Chief Minister Nitish Kumar.
This is not the first time the actor has donated such a big amount to a state.
Earlier this month, Khan donated the same number of amount to the flood victims in Assam and Gujarat.
Chief Minister of Assam Sarbananda Sonowal took to Twitter to thank Aamir Khan for the generous contribution.
Over one crore seventy lakh people have been affected by the floods in Bihar. The death toll has mounted to 415, with 21 districts being affected by the deluge.
22 lakh people of the district Darbhanga have been affected by the floods.
Power Sub-Station-Poria in Darbhanga district has been flooded, electricity in many villages has been affected badly.
According to the locals there, Power sub-station has been flooded for the last 15 days due to which power supply has hit badly.
District Magistrate of Darbhanga, Chandrashekhar has given the assurance that the alternative arrangements are being being in 1-12 villages and the situation will get better in next one or two days.
The situation continues to worsen in Muzaffarpur, Samastipur and Darbhanga districts. Urban areas of Muzaffarpur have been deluged following the overflow of water in the Tirhut Canal
Rail traffic on Darbhanga-Samastipur section continues to be disrupted.