It happened on Grand National Day at the Ladbrokes on Victoria Road.
There's been an armed robbery in Woolston.
A man went into Ladbrokes on Victoria Road and demanded money with what looked like a handgun, on Saturday evening.
Staff refused to hand over any money and the man got away on a black mountain bike.
The man is descibed as:
DC Danielle Daltrey who is investigating said: "This was a busy day for the shop as it was Grand National Day. Although the man left empty handed, this is a serious incident which resulted in staff being left scared and shaken.
"We’d like to hear from anyone who was in Ladbrokes around this time, or who saw a man cycling away from the shop.
"If you do recognise the man in the CCTV images please contact us.”
Anyone with any information is asked to contact DC Daltrey on 101 or via the website www.hampshire.police.uk/internet/contact/message_officer.htm
You can also call the charity Crimestoppers line on 0800 555 111 where information can be left anonymously.
Minicom users should call 01962 875000.