Strathclyde Police are appealing for information following the theft on Sunday.
The elderly man was at home in Bonnyton Road, Kilmarnock when two men entered through an unlocked door, between about 1pm and 3pm. The men, who had Irish accents, grabbed him, threatened him with what appeared to be a knife and robbed him of a four-figure sum of money that was in his house.
The victim contacted police who searched the area, but there was no trace of the suspects. It is thought they may have left in a white-coloured van or pick-up along Bonnyton Road.
Detective Sergeant Russell Andrew from Kilmarnock CID said: "To target a man of 83 years in his home, push him about and steal his belongings is absolutely despicable."