Five children are among the dead. There are fears the death toll could rise further as many people are missing.
Several houses were washed away and people became trapped in the district of Kottayam in Kerala state.
Video from the area showed bus passengers being rescued after their vehicle was inundated.
Kottayam and Idukki are two of the worst affected districts in the state, where days of heavy rainfall have caused deadly landslides.
Swollen rivers have also washed away bridges connecting many small villages.