The deceased included two children and two women as their houses on slopes of the hills were swept away by flash floods caused by heavy rains battering the area since Sunday, police said.
Casualties were reported from Kunchithanni, Karimannur and Urumbidam.
Roads linking the hilly district with the rest of the state have been blocked by boulders and rubble crashing down in landslides.
Idukki MLA Roshi Augustine said relief works are hampered by disruption in road traffic.
However, the district administration has geared up to meet the situation with the help of local people, Augustin said.