The 80-year-old man was asleep at his home in Raithburn Avenue, Kilmarnock, East Ayrshire, on Sunday when he was disturbed by the sound of a break-in.
The suspect, who carried out the raid at about 9.20pm, demanded money and then ran off with a two-figure sum of cash towards Onthank Drive.
Police do not yet have a description of the suspect but officers are carrying out inquiries in the local area and checking CCTV footage as part of their investigation.
They are urging anyone who saw anything suspicious, or who has any other information about the incident, to contact them.
Detective Constable Scott Moore said: “To target an elderly man within his own home is cowardly and totally unacceptable. He must be caught.
“Luckily, the elderly man was uninjured, however he has been left extremely shaken by this robbery.”