The collection of 913 coins, declared as treasure by a coroner on Thursday, was found hidden beneath the instrument’s keyboard at a school in Shropshire as it was being retuned.
It is reported the hoard could be worth hundreds of thousands of pounds, though it still has to be formally valued.
Experts from the British Museum say the coins range in date from 1847 to 1915 and include 633 full sovereigns and 280 half-sovereigns.
They were found neatly stacked in several hand-stitched bags and would have been worth around £773 at the time they were hidden, well above the average house price of £619.
The majority of the coins date from the reign of Queen Victoria and were found to be 91.7% pure gold.
At an inquest into the discovery, coroner John Ellery ruled that the gold coins were treasure but said: “We simply do not know how they came to be concealed.”
An international media appeal as well as local research failed to shed any light on who put the 6kg hoard into the piano.
The instrument previously belonged to Graham and Meg Hemmings, from Saffron Walden in Essex, but they donated it to Bishop’s Castle Community College in Shropshire after moving nearby in 2015.
The couple had owned it for 33 years without any knowledge of its secret.
The hoard was discovered by tuning technician Martin Backhouse, 61, who initially thought the “gobsmacking” discovery was “moth repellent” before realising it was too heavy.
After slitting the stitching with his penknife, he realised there was “rather a lot of gold in this”.
Both Mr Backhouse and the school could be in line for a windfall from the hoard’s sale.
Mr and Mrs Hemmings said they did not regret donating the piano to the school.
Mrs Hemmings said: “The sadness is, it’s not a complete story. They’ve looked and searched for the people and they unfortunately haven’t come forward.
“It’s an incomplete story – but it’s still an exciting story.”
During the hearing, Peter Reavill, from the British Museum, said one of the packets contained an old Shredded Wheat advertising card, meaning the hoard was probably “repackaged” sometime during the Great Depression era.