The trustees of Bodelwyddan Castle in Denbighshire have put the 125-year lease they hold on a large part of the property up for sale.
Bodelwyddan Castle Trust (BCT), which runs the castle, museum and art gallery, near Rhyl, wants to use the money it will raise to continue its public education role and allow access to its collection of Welsh art by linking to a charity with a similar ethos.
The decision to sell the remaining 99 years of the lease to the highest bidder and close in 2019 follows the withdrawal of a £144,000 support grant from Denbighshire County Council.
In a Facebook post on the castle's official page, management said the house would close to the public "whilst negotiations are in place".
They thanked "trustees, staff and volunteers" for their support. Dr Helen Papworth, Chair of trustees, described the "loyal staff and volunteers" that work at the attraction.