The tiny island group of about 40 residents had managed to remain infection-free by completely closing itself off to visitors.
Border restrictions, imposed in March 2020, began easing from April.
One of the island's residents who left for medical treatment tested positive on their return.
The Times UK reports the patient is isolating - along with those they travelled with.
The Pitcairn Islands, which are to the east of French Polynesia, are a British overseas territory administered from New Zealand.