At least 101 people died following several days of incessant rain, the country's Disaster Management Center (DMC) said.
At least 111,900 people are living in temporary shelters.
As many as 47 bodies have been recovered from two landslide sites and over 180 people are still missing, the Sri Lankan Red Cross Society says.
Rescuers are searching for dozens of other people reported missing after mud and rock buried residential areas, the nation's Disaster Management Center said.
The death toll is expected to go up as rescue teams reach some of the worst-hit areas, authorities said.
For the rest of the week, Sri Lanka will likely be experiencing scattered rainfall that occurs just a couple weeks before the monsoon season, CNN meteorologist Sean Morris says.
Various nations, including India, Australia, Nepal and Japan, are bringing in aid supplies such as blankets, water purification tablets and drinking water.
Sri Lanka's military is airdropping food and water to affected areas, authorities said.
Some of the worst-hit areas include the suburbs of Colombo, where the Kelani River passes through before ending in the Indian Ocean. In Wellampitiya suburb, police said water levels stood at over 8 feet after four days of rains.
Most festivities to mark the birth, death and enlightenment of Lord Buddha have been put on low-key status. Free-food centers for those viewing decorations on city streets have been drastically reduced to divert funds to the victims.
"We are now in control of the situation. We appreciate the prompt response of many countries," Finance Minister Ravi Karunanayake said.
The government is advising those displaced not to return home in the next three days, according to Anura Priyadarshana Yapa, the minister of disaster management.
Food distribution will continue, he said.