A 19-year-old man was robbed as he withdrew money from a cash machine outside the Nat West Bank in Terminus Road, Eastbourne.
The victim was struck on the head and had £30 stolen at around 10.15pm on Friday (January 9) by a man wearing a grey hooded top and dark blue jeans.
Despite being dazed he ran after his attacker but was forced to give up the chase when the suspect mounted a BMX bike left in a nearby street and rode off.
A 20-year-old man was later arrested on suspicion of carrying out the robbery and bailed until March 23 while enquiries continue.
Anyone who saw what happened or who may have other information is asked to email email@example.com or phone 101, quoting serial 1534 of January 9.
Alternatively they can call the independent charity Crimestoppers anonymously on 0800 555 111.