The director-general of the ADB's Pacific Department said Gita was one of the most powerful cyclones in Tonga's recorded history and it had directly affected almost 80 percent of the population and damaged or destroyed around 2,000 homes.
Carmela Locsin said the grant will go towards restoring critical services, like water and electricity, clearing and disposing of rubble, and providing transitional shelter.
The ADB said along with damage to property, including the destruction of the historic Parliament, an estimated 80 percent of the electricity network in the Nuku'alofa area is damaged.
The bank said restoration of the electricity network could take more than seven weeks.
The ADB had already given $6-million in the immediate aftermath of the storm.