The victim was walking through Tower Hamlets Cemetery Park in Bow Common Lane when a youth followed her inside and grabbed her at about 3.30pm on Friday.
“He sexually assaulted her by touching her,” a Tower Hamlets police spokeswoman said. “He then made off towards Devons Road.
“The suspect is described as an Asian male aged approximately 16 years, approximately 5ft 10in.”
Police believe the youth was carrying a blue Adidas bag with a white Adidas logo, and was wearing dark trousers, black shoes, and a blue quilted jacket.
Anyone with information about the assault should contact Det Con Shoaib Iqbal at Limehouse Police Station on 0207 275 4694, quoting crime reference number 4207309/13, or call Crimestoppers anonymously on 0800 555 111.