Officers received a call from the owner of a property in Browning Road, Worthing, to report a break in at his flat at around 1.15pm on Friday (February 2), police said.
The victim was working from home when he noticed a man entering his living room, a spokesman said.
When spotted by the suspect, he was sprayed in the face with a substance believed to be pepper spray, according to the spokesman.
Police describe the suspect as a mixed race man, 5ft 7ins and with short dark hair.
He was wearing a dark blue jacket, a scarf covering the majority of his face and was carrying a rucksack, added the spokesman.
Detective Constable Chris Bishop said: “This was a very alarming and distressing situation and he victim is lucky to have not sustained any serious injuries.
“We believe the suspect may have climbed up the side of the property to gain access.
“If you witnessed a man acting suspiciously in the area around this time or you have any information please report it to us immediately.”
Nothing was stolen in the incident, police added.
Anyone with information, call 101 quoting reference 537 of 02/02.