The Â£1 bank note, from 1819, was stolen from Padstow Museum in Cornwall in 1984.
It has been returned to the museum by airmail without any explanatory note or return address but bearing stamps from St Lucia.
Museum chairman John Buckingham said the tale was "worthy of a TV drama".
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"I was very surprised," Mr Buckingham said. "It's just the length of time it's been since it went missing.
"It's a real puzzle. Who's had it all that time?
"It's not hugely valuable, but it's valuable from a local history point of view. I'm just very pleased that the note is back at the museum."
The note had been issued by the Padstow Bank of Thomas Rawlings - a wealthy merchant in the town in the early 1800s, who set up his own bank.
The 1984 museum ledger lists the banknote as "stolen".
The Rawlings family lived in a mansion Saunders Hill, which is where Lawn Car Park is now, close to Padstow Social Club.
"It marked the pinnacle of the Rawlings family fortunes which seem to have collapsed in 1820 when Thomas Rawlings died aged 62," Mr Buckingham said.
A 200-year-old bank note stolen from a museum 33 years ago has been returned anonymously after being posted from the Caribbean.