The announcement came after the ADB's country director Yong Ye met Pakistan's Finance Minister Ishaq Dar in Islamabad.
"ADB will provide flood relief support to Pakistan to the tune of $US2.3 to $2.5 billion," the ministry said in a statement.
The money includes $1.5 billion for one of its aid programmes known as Building Resilience with Active Countercyclical Expenditure (BRACE), which will be placed before the ADB board for approval during October, it said.
The floods caused by abnormal monsoon rains and glacial melt have submerged huge swathes of the South Asian country and killed nearly 1,700 people, the majority of them children and women.
The United Nations on Tuesday revised five-fold its humanitarian appeal for Pakistan, to $816 million from $160 million, as a surge of water-borne diseases and fear of growing hunger posed new dangers, which the World Health Organization (WHO) has termed a "second disaster".