Police said a man wearing a balaclava, described as possibly being like a “spiderman mask”, forced his way into the house on Saturday at about 20:55.
He brandished the knife before making off with a quantity of cash.
Detectives called the robber “despicable” and urged anyone with information about him to come forward.
They also believe he may have tried to gain access to other properties before the robbery.
He was described as being about 6ft tall, of medium build and between 20 to 30 years old. He was wearing a dark hooded top, dark trousers and the balaclava.
The pensioner was not injured during the incident.
Det Insp Jim McLauchlan said: “An elderly man has been subjected to a terrifying ordeal within his own home and it absolutely vital that we trace the despicable individual responsible.
“We understand that the suspect may also have attempted to get into other properties nearby and so I would appeal to anyone who may have noticed someone acting suspiciously in the area yesterday evening to get in touch.”