The attack happened at about 0330 BST on Tuesday next to the Global Cafe in London Street, Reading, Berkshire.
The woman had been in the Brannigans pub with friends when she began talking with a group of men.
She left with one of them who then dragged her into the alleyway. He is described as white, in his late teens, with dark hair and of medium build.
The man was wearing blue denim jeans and a black short-sleeved top.
Thames Valley Police are appealing for anyone who may have been in Brannigans, or saw a man fitting the description in the London Street area, to come forward.
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