Sri Lanka's navy on Sunday arrested six Indians trying to smuggle 13.5 kilos (29.7 pounds) of heroin across the narrow strip of sea dividing the two countries, a spokesman said.
The suspects were disguised as fishermen, navy spokesman Chaminda Walakuluge said.
"Drug smugglers operate disguised as fishermen," Walakuluge told AFP. "We are on alert for drugs coming in."
He said the navy has seized a total of 130 kilos (286 pounds) of heroin and cocaine and 2,140 kilos (4,708 pounds) of cannabis in the past year and arrested a total of 19 Indians during the same period.
In 2014, Sri Lanka sentenced five Indian fishermen to death for drug smuggling, but they were later released into Indian custody.
The killing of an Indian fisherman last month along the maritime boundary led to widespread protests in the southern Indian state of Tamil Nadu.
India and Sri Lanka are separated by the narrow Palk Strait, which is a rich fishing ground.