Tanera Mor, the largest and only inhabited one in the Summer Isles archipelago, is being sold by the Wilder family, who bought it in 1996.
The asking price has been reduced by £500,000 to £2 million for the island, lying off Wester Ross.
The 800-acre isle was the setting for Frank Fraser Darling's 1943 book Island Farm, and related to the 1973 film The Wicker Man, set on the fictional island of Summerisle.
A spokeswoman for estate agents Savills said: "Once- windswept paths are now sheltered by semi-mature trees and the approach to the island is softened by the woodland.
"The island is currently managed and run as a successful tourist enterprise, incorporating a number of different facets."
The name Tanera is thought to derive from the Norse word for "haven".
The seller said: "Indeed, for those seeking the seclusion and privacy that only island ownership can bring, whilst being conveniently situated for the mainland, Tanera Mòr will be a strong contender.
"Buyers who are seeking adventure will find world-class sailing, kayaking and diving in their own playground.
"The island is a paradise for lovers of nature, who will find an unusual and diverse little world to nurture."
The family earlier held discussions with the Coigach community, which includes the mainland village of Achiltibuie, to establish whether locals would be interested in buying the island before it was put on the open market.
As well as being home to otters and seals, the island's natural harbour formerly provided a safe stopover on northward sea journeys before the nearby mainland port of Ullapool was established.