Alice Head-Rapson, an ONWP trapper, said: “This incursion response was a test of our skills, but everybody at the project stepped up to meet the moment.
“It’s still important we guarantee with complete confidence that the stoat we’ve trapped was the only one, but our achievement here is a big deal and we hope that will help to keep nature safe across all of Orkney.
“Working with the island community has been a privilege, and we’re really grateful for all the support the volunteers there have shown for native wildlife.”
Rousay is the first of the outer islands where a stoat has been spotted.
Though it was not clear how the creature ended up there, the ONWP said that the island was within swimming distance – 1.8 miles (3km) from the mainland.
Monitoring will continue for two years before the island can be declared stoat-free.