Tanera Mor is the largest and only inhabited island in the Summer Isles archipelago 1.5 miles off the north-west coast of Scotland.
The 800-acre island is managed by Lizzie and Richard Williams, who took it on from Mrs Williams’s family, the Wilders.
The Wilders bought the island in 1996 after selling their dairy farm in Wiltshire.
Mrs Williams said it was time to give someone else the “privilege of looking after the amazing place”.
Talks with the local community over a buyout were held but it has now been put on the open market after the Coigach Community Development Company decided not to pursue the sale.
Mrs Williams said: “We hope that whoever owns the island in the future will enjoy the same warm and co-operative relationship with the community that we have had for the past 17 years.”
Estate agent CKD Galbraith, which is now handling the sale, said the community group welcomed the opportunity to buy the island but was currently working on renewable energy projects.
CKD described Tanera Mor as a “flourishing tourist enterprise and superb family residence”. It has nine residential properties, a café, post office and three jetties.
Company spokesman John Bound said: “We expect to receive a lot of interest.”