The man, who is in his 70s, confronted the criminal during an incident at a bookies in South Shields.
Officers were called to the William Hill, on the town’s Brunswick Street, after a raider entered the store in possession of a knife.
The masked robber threatened a member of staff and demanded money before leaning over the counter and grabbing nearly £100 in cash.
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This happened on Friday, November 15.
But as the robber attempted to leave the shop, along with a second man who was holding the door, he was confronted by a heroic customer.
The customer threw a chair at the robber after a brief struggle, which caused him to drop his knife and spill a quantity of coins over the floor. The robber then fled the scene.
Nobody was injured, but the member of staff and the customer were both left shaken by the ordeal.
Officers are appealing to the public for further information, while the pensioner has been thanked by Northumbria Police for his selfless actions.
Detective Constable Michael Wilson said: “There can be little doubt that the customer went far above and beyond anything that was expected of him during this frightening ordeal in a bid to apprehend the robber.
“We would never encourage members of the public to put themselves in harm’s way. However, we thought it was important to contact the customer following this incident and thank him personally for putting his own safety selflessly to one side in a bid to stop the robbery.”
He added: “We understand that two men entered the store on the night in question – one of whom stole the money from the counter, and a second male who waited by the door and then fled alongside the other offender.
“They were both wearing masks and ran through the cut onto St Mark’s Way after leaving the shop. If anyone saw these men either before or after the robbery, please come forward and let us know.”
Anyone with information is asked to contact 101 quoting reference 1091 151119 or Crimestoppers anonymously on 0800 555 111.