The man demanded cash when he went into the bank in Old Mill Square soon after 11am.
A police spokesman said: “He intimated that he had a weapon, but none was seen.
“He is described as black, in his 20s, wearing a dark parka jacket and wearing a hat or a hood. He was very quietly spoken.
“Anyone who witnessed the incident or who may have seen the man as he left the bank is asked to report online or call 101 quoting serial 420 of 25/04.”
Police are still on the scene of the robbery.
Meanwhile, bank officials have apologised for closing the branch following the robbery.
A notice posted in the window states: “Sorry, but due to circumstances beyond our control we are unable to open this branch at the moment.
“We are doing everything we can to get things up and running again.”