Barrow Hospital near Bristol goes on sale this weekend as part of plans to improve mental health services in Bristol and the surrounding area.
The Avon and Wiltshire Mental Health Partnership NHS Trust says it intends to sell the site as a single lot.
The hospital is due to close to patients next summer when services transfer from Barrow Gurney to Callington Road in Bristol.
All of the proceeds from the sale will be reinvested, the trust says.
Spokeswoman Ann-Marie Llewellyn said: "In line with our guidelines the sales process has to be transparent and has to be consistent and everybody will be treated fairly.
"We don't know who is going to be interested in the site but we're hoping to attract as much interest as possible."