The navy and air force are searching for 26 fishermen who have been missing at sea since the monsoon hit early Saturday, said a spokesman for the Colombo-based Disaster Management centre, Sarath Lal Kumara.
The bodies of 22 fishermen have so far been found, while the body of another person was discovered on land, Kumara said.
"The weather has improved during the day and the rescue operations are underway to look for survivors."
A government minister said at least 21 injured fishermen had been rescued and admitted to hospital and assistance was being offered to bereaved families.
Sarath Kumara Gunaratne, deputy minister of fisheries and coordinator of disaster relief, said most of the victims were fishermen using very small boats that operated close to the coastline.
On Saturday officials confirmed the deaths of five people due to the monsoon which hits annually, bringing much-needed rain, but also frequently causing loss of life and damage to property.
Security forces were deployed to clear fallen trees and power lines strewn on several key highways, the military said.
A six metre whale washed ashore in Colombo on Saturday while a stranded giant sea turtle was rescued by locals at a suburb of the coastal capital, police said.
Authorities opened the sluice gates of four major hydroelectricity reservoirs in central Sri Lanka late Saturday to prevent damage to dams.
Seven people were killed in the country last month when a tropical cyclone brushed the eastern coast.