An aerial and vessel survey of the Limmen Marine Park and the Limmen Bight Marine Park in the Gulf of Carpentaria found eight seagrass species growing in shallow water to a depth of 20 metres.
They provide critical habitat for vulnerable dugong and turtle populations as well as fish, crabs and other marine species.
Marine biologist Rachel Groom said the study added to the knowledge about migratory and threatened species that rely on seagrass habitats.
"We are now mapping and describing the seagrass habitats that underpin ecosystem function and are critical habitats for dugong and turtle," Dr Groom said.
The 2250sq/km survey was conducted over two weeks using helicopters to map sea floor habitats exposed at low tide and boats for deeper waters.
Marine biologist Alex Carter said the team had surveyed more than 3000 seagrass sites across Marra and Yanyuwa Indigenous sea country.
"Seagrass meadows extend from shallow sandbanks next to mangroves to waters 20 metres deep at the edge of the marine park," she said.
The research is a collaboration between Mabunji li-Anthawirriyarra Land and Sea Rangers and James Cook University and Charles Darwin University scientists and the NT Parks and Wildlife Commission.
Senior li-Anthawirriyarra sea ranger Shaun Evans said the mapping project was important for setting a foundation for future seagrass monitoring.
"We need to know what resources we have and where they are located," he said.
"These habitats support turtle, dugong and fish that are significant to Marra and Yanyuwa people, knowing they are healthy is important for us".