Michael O'Grady, 64, from Northern Ireland, was last seen walking in the Ben Alder and Loch Ossian areas in the Highlands on December 22.
He has not returned to his car, which has been parked in the area since Christmas.
He is 5ft 6ins and slim. He has grey hair and is said to be an experienced hillwalker.
Police want to speak to anyone who has been in the area over the Christmas holidays and may have seen him.
Mr O'Grady, from Irvinestown, Co Fermanagh, was last seen on December 22 having left the Loch Ossian youth hostel intending to walk to Culra Bothy.
It is thought that he may have been seen by a group of five hillwalkers that day near to Ben Alder.
Police Scotland are eager to speak with anyone who may have seen or spoken to him that day and asked anyone with information to contact them
A police spokesman said: "Although an experienced hillwalker, there has been no contact made with him for several days and we are keen to speak with any persons who may have been in the area, particularly the area around Ben Alder, who may have seen or spoken to the male.
"Anyone with information can contact Police Scotland on 101 or speak to any police officer."