Three men wearing balaclavas used a back patio door to force their way into the property in The Wroe just before 11pm on Sunday.
The men demanded money from the victim, a man in his late 60s, who was threatened and hit on the arm. They got away with more than £1,000 in cash.
Police are appealing for more information.
Det Con Emma Cross, who is investigating the incident, said: “The victim suffered a slight bruising and was understandably left extremely shaken by what happened.
“A number of police resources including armed units and the NPAS helicopter (National Police Air Service), were deployed to the area to carry out searches which proved unsuccessful.
“We’re keen to hear from anyone who may have witnessed anything unusual in this area late that night or has knowledge of those responsible.”
Anyone with information should contact DC Cross at King’s Lynn CID on 101 or call Crimestoppers anonymously on 0800 555 111.