The man went into a branch of RBS in the Craigentinny area of Edinburgh yesterday and demanded money from the cashier.
He escaped with a large sum of money and made off on foot into Restalrig Avenue.
Police said the man’s clothing and cash may have been contaminated with red dye after a security device was activated.
They appealed for information about the incident, which happened at around 4.45pm.
Detective Inspector Alan O’Brien said: “This robbery was a traumatic experience for the staff working in the branch, although thankfully no-one was hurt.
“We are appealing for anyone who witnessed this robbery, or recognises the description of the suspect, to contact police immediately.
“The suspect and the money are likely to have been contaminated with red dye, so if anyone comes across anything suspicious, they should also contact police on 101 or Crimestoppers on 0800 555 111.”
The suspect is described as white, of medium or slim build and around 6ft 3in.
He is aged around 25, clean shaven, and was wearing a blue baseball cap, a light grey body warmer with hood attached, dark baggy trousers, dark footwear and black gloves.
Police said a “substantial quantity” of money was taken but would not comment on the figure.