The police of Limon, a province about 170 kilometres east of the capital San Jose, identified the victim as 26-year-old Jairo Mora Sandoval.
Sandoval's body was found face-down and handcuffed next to the car he was driving, police told AFP.
Authorities declined to provide more details on the case, saying only that an investigation was under way.
The US Embassy in Costa Rica called the killing "senseless" in a post on its Facebook page that also hailed Mora as a committed environmentalist.
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The others are well and managed to escape and notify police, who found Mora on the beach, Chacon said.
Mora monitored nesting turtles, recording the number of nests and trying to rescue eggs, Chacon added, noting the group's program was entirely volunteer-based and dependent on donations.
Mora had complained in a La Nacion newspaper report published in April that the area's environmentalists were threatened by criminal organizations apparently because those engaged in looting turtle nests are linked to drug traffickers.
He described Limon as a no man's land.
"We have all received threats so that we leave when all we want is to protect the turtles," he added.