The 64-year-old Northern Irish man was walking near Loch Rannoch, Perthshire, but has not returned to his car which has been parked in the area since Christmas.
He is 5ft 6in with a slim build and 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.
A police spokesman said: “Although an experienced hill walker 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.”