Police said the 200-kilogramme (440-pound) haul -- worth more than two billion rupees (USD 13.3 million) on the street -- was found stashed inside plastic bags in a car north of Colombo.
It was the biggest heroin seizure since 2013, when 260 kilos were taken off the street, police said.
The seizure came after a drugs agent was killed Tuesday in a shootout that also wounded several other officers.
"Our latest detection shows we will not be deterred by attacks against our men," police said in a statement.
Last June, anti-narcotics authorities seized 90 kilos of cocaine concealed in sugar containers from Brazil. Two months earlier police detained 14 foreigners after seizing 110 kilos of heroin worth USD 7.5 million from an Iranian fishing trawler.
Disclaimer: No Business Standard Journalist was involved in creation of this content