The unnamed supporter, who died last year at the age of 93, made a bequest of £108,725.
The donation is the largest single cash sum from an individual ever received by the fans’ movement, which has been working to bring the club into supporter ownership.
Foundation chairman Brian Cormack, who announced the cash boost, said: “This was an amazing heart-warming gesture by this gentleman.
“In his will, he stated that part of his estate was to go to an organisation which promoted the aims and objectives of Heart of Midlothian Football Club. We should also mention that he requested that this bequest be made anonymously.
“We received the funds on June 24 and are discussing how this legacy might best be used.
“The entire bequest is being passed on from us to the club.
“On behalf of all Foundation members, I would like to say how very touched we are by this supporter’s generosity.
“It means a huge amount to the Foundation and the club, and is another reminder of the love that supporters have for Hearts and the lengths they go to help the club prosper. We are very grateful.”
Hearts owner Ann Budge said: “This was a truly wonderful gesture from someone who clearly had a deep affection for the club and believed in the values on which this club has been built and again aspired to promote and stand by.
“I would like to extend my very sincere thanks for this bequest. Given its background and its significance, we will take time to establish an appropriate way to make best use of these funds and will make an announcement shortly on this. As Brian has also commented, we are – as ever – astounded by the generosity and commitment of fans of the club.”
It is understood that the 93-year-old man split his estate four ways and that charities may also have benefited.
The Foundation provides working capital to the club and is funded by monthly financial contributions or pledges from members.