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 with floodwater.
Kottayam and Idukki are two of the worst affected districts in the state. Days of heavy rainfall has also caused deadly landslides.
Military helicopters are being used to fly in supplies and personnel to areas where people are trapped, officials said.
Thousands of people have been evacuated and more than 100 relief camps have been set up across the state, Kerala’s Chief Minister Pinarayi Vijayan said on Sunday.
Prime Minister Narendra Modi tweeted on Sunday, saying that he had spoken to Mr Vijayan about the situation. “It is sad that many people have died due to the heavy rains and landslides in Kerala. My condolences to the bereaved families,” Mr Modi said.
Fishing boats are being used to evacuate survivors trapped in Kollam and other coastal towns, as sections of road have been swept away and trees uprooted.
It is not uncommon for heavy rainfall to cause flooding and landslides in Kerala, where wetlands and lakes that once acted as natural safeguards against floods have disappeared because of increasing urbanisation and construction. – BBC