Henry Patterson, 83, from Bishopbriggs, East Dunbartonshire, was found by specialist search officers in a wood near his home town.
Police issued a public appeal after he was reported missing on Monday.
His family were said to be “extremely concerned” as he has early-stage dementia and does not normally leave the house on his own.
Officers said on Thursday he was found in a wood near to Cawder golf club and the Forth and Clyde Canal.
He has been taken to Glasgow Royal Infirmary for treatment and medical staff describe his condition as critical.
Chief inspector Craig Smith said: “I would like to thank everyone who took the time to share our posts to help find Mr Patterson.
“Unfortunately due to the length of time he has been missing and his age, he was found very unwell and is currently within hospital receiving treatment.
“Mr Patterson’s family are grateful for the help and support they’ve received from the local community and beyond.”