David McCartney, 74, told his family he was going on a walking expedition and expected to be around four to five hours - but he has not been seen since.
North Yorkshire Police have drafted in Upper Wharfdale Fell Rescue Association and the RAF to help in the search for David.
David was last seen when he left home in Keighley on October 18, and said he would be heading to Craven for a hike.
He is described as wearing a blue kagool, red trousers, a orange and brown striped scarf and he was carrying a red and black rucksack.
A statement from North Yorkshire Police said: "We are currently searching for David McCartney, 74, who was last seen at his home address in Keighley on Monday.
“David told his family he was heading on a walking expedition and would be approximately four to five hours. He travelled to the Craven area in a black Volvo 62 plate V60.
“We are currently carrying out enquiries to locate him but are growing increasingly concerned for his welfare. To assist with the search, we have called in the Upper Wharfdale Fell Rescue association and the RAF who are assisting with a helicopter.”
“David is described as wearing a blue kagool, red trousers, a orange and brown striped scarf and he was carrying a red and black rucksack.”
“Anyone who has seen David, or his vehicle, or has information that could assist us, is asked to call 101. If you have an immediate sighting please dial 999”