It follows devastating floods in Nepal this month which killed at least 225 people, with water adding to already swollen rivers in India and Bangladesh.
Deadly floods and landslides are common across South Asia during the monsoon season from June to September but experts say climate change is making them worse.
In India's northeastern Meghalaya state, "relentless rainfall-induced floods and landslides" had caused a swath of destruction, the chief minister's office said.
"We are saddened to inform that the death toll from the landslides and floods in the Garo Hills region has increased to 15," it said in a statement, with the deaths recorded since Friday.
Those killed include a teacher and his son, who drowned when floods swept away their car.
In neighbouring Bangladesh -- a low-lying nation with large areas made up of deltas where the Ganges and the Brahmaputra wind towards the sea -- five people have been killed since Saturday, the disaster management ministry said.
At least 20,000 people have been forced to flee their homes, it added, but water levels are beginning to drop.
"Floodwaters have started to recede... excessive rain and the water from upstream caused this flood," disaster ministry official Nazmul Abedin said.