The serious sexual assault happened in the Peatlands Park area of Dungannon on Wednesday, April 27.
Detective Inspector Lynne Knox said: “Police received a report that at approximately 2.45pm as a woman was walking her dog in the Peatlands Park area she was sexually assaulted by an unknown assailant.
“The male made off from the scene on what has been described as a black mountain bike.
“He is described as being approximately 6’ tall, aged in his 20’s, clean shaven, with a chubby face, and droopy eyes.
“He was wearing all black clothing, a high visibility jacket and a cycling helmet”.
DI Knox continued: “This was an appalling attack and police are pursuing all investigative lines of enquiry.
“I would appeal to anyone who may have witnessed this assault or observed anything suspicious to contact Detectives at the Rape Crime Unit on the non-emergency number 101.
“Alternatively, if someone would prefer to provide information without giving their details they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.”