A woman and two men were approached by the man in Western Place, south of Rowlands Road at about 3.10pm on Tuesday, April 20.
The man produced a knife and threatened the two men.
He took a watch from one of them and a wallet with 40 cash and cards from the other man. The two victims were accompanied by a woman in her twenties but she was not robbed. Nobody was assaulted or injured.
Detectives are investigating the robbery.
The thief is said to be in his mid-20s to mid-30s, at least 6ft tall, of medium build with a bald head and a thin dark moustache.
He was wearing a short-sleeve dark t-shirt, dark blue jeans and dark shoes.
Anyone with information should contact Sussex Police on 0845 60 70 999.
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