The man had just withdrawn cash from the Halifax machine on Witton Street, when he was attacked, at around 9.20pm on Sunday.
He suffered a blow to the head from a cylindrical bar but managed to hang on to his money after a scuffle.
The attacker fled up Albion Road towards Sainsbury's and was joined by a man.
Detective Constable Kevin Chambers said: "The man who was attacked wasn't able to get a very good look at the person - but we strongly suspect they were female."
The figure was aged around 16, and about 5ft 4ins in height.
She was wearing a distinctive blue baseball cap with a red peak and the word 'Budweiser' written on it in red.
DC Chambers added: "I'd like to appeal to anyone using either the Halifax or Abbey National cash machines at that time to contact us if they saw either a lone female, or a couple, acting suspiciously.
"I am also appealing for anyone who was in Albion Road to come forward for the same reasons."
You can ring DC Chambers on 01244 613258.
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