The discovery was made in Shropshire in December last year when the piano's new owners had it re-tuned and repaired.
Shrewsbury Coroner's Court decided the find qualified as treasure, which means ownership now lies with the Crown. It will be offered for sale to museums, the BBC reported.
The tuner who found the hoard and Bishops Castle Community College, which owns the piano, will share a reward.
There are 913 gold sovereigns and half sovereigns dating from 1847 to 1915, from the reigns of Queen Victoria, Edward VII and George V.
The gold coins have a face value of over 770 pounds - the equivalent of roughly 500,000 pounds today.
The gold coins were found under the keyboard of the piano, carefully stitched into seven cloth-bound packets and a single leather drawstring purse, and amount to more than 6kg of gold bullion.
The inquest revealed the original owner and their heirs remain unknown.
The hoard's market value will be decided by an independent Treasure Valuation Committee at the British Museum.
The upright piano was made by Broadwood and Sons of London and sold to two music teachers in Saffron Waldon, Essex in 1906.
After that, the piano's history is unknown, until 1983 when it was purchased by the Hemming family in the Saffron Walden area.
They moved to Shropshire in the 1990s and donated the piano to the college to help its students learn to play music.
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