Two raiders threatened bank staff with a gun before making off with hundreds of pounds.
The men entered Barclays Bank, in Lower Street, Pulborough, and used what appeared to be a black semi-automatic pistol.
A police spokesman said the men fled on foot down an alleyway towards Carpenters Meadow.
The first man is described as white, 5ft 10in tall, of proportionate build, aged between 20 and 40 years, tanned complexion, wearing a black baseball cap covering a shaved head, wrap-around sunglasses and a coloured T-shirt.
The second was white, 6ft to 6ft 2in, stocky build, aged in mid 40s, short blond hair and wearing a light coloured top.
Anyone who saw the men acting suspiciously or leaving the bank should ring Horsham CID on 0845 6070999.
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