Marcus Nash alerted birders nationwide after seeing the citril finch in Burnham Overy on Sunday morning.
The bird normally lives in mountainous areas of Europe including the Alps and the Pyrenees.
Mr Nash said: "I've seen them before in the Alps, so immediately knew what it was."
The male bird remained at the dunes during the day and was spotted again on Monday.
Paul Stancliffe, from the British Trust for Ornithology, said the only other recorded sighting of the bird in the country was on the Shetland Isles, in 2008.
"It's just an incredible record," Mr Stancliffe said. "It doesn't normally move very far. It must have got caught up in a weather front."
David North, from Norfolk Wildlife Trust, said the area was a "mecca" for wild birds and continued to amaze.
"There are always surprises but not many birders would have guessed they would see a citril finch," he said.
Mr North urged visitors to keep to footpaths, to protect nesting birds.
Hundreds of bird watchers have travelled to Norfolk in the hope of seeing a bird that has been spotted for only the second time in the UK.