Detectives are appealing for information after the suspect entered the Coral bookmakers on Woodgrange Drive, Southend, at around 8.45pm on Thursday, December 18.
He stole hundreds of pounds and injured a member of staff with the sword.
The victim, a man in his 50s, suffered a nasty wound on his chin and required stitches at hospital.
He is now recovering at home but is very shaken by the attack.
Detective Sergeant Phil Butt said: "This is a very serious offence and the level of violence is of great concern.
"This man needs to be found."
The suspect is described as a white man, aged 18 to 22, and between around 5ft 8ins and 5ft 9ins tall.
He was wearing a green camouflage jacket, a dark coloured beanie hat and a pink scarf over his face.
Anyone with information is asked to call detectives at Southend on 0300 333 44 44 or Crimestoppers, in confidence, on 0800 555 111.