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.
Days of heavy rainfall in Kerala has caused deadly landslides and the Indian military has joined rescue efforts.
Helicopters have been used to fly in supplies and personnel to areas where people have been trapped under debris by the landslides, officials said on Sunday.
In one tragic incident, a family of six - including a 75-year-old grandmother and three children - were confirmed dead after their home in Kottayam was swept away, news agency PTI reported.