A 30-year old woman was assaulted as she walked home from a Long Eaton nightclub with a man in the early hours of Sunday morning.
Derbyshire Police said the victim left Manhattans, in Clifford Street with the man at around 3am.
She was walking along Tamworth Road with him when he led her into the grounds of the library in Friar Street and assaulted her.
The man is described as white and in his mid-20s.
Part of the grounds were cordoned off on Sunday while forensic teams examined the scene.
Anyone who witnessed the assault or has information should call the police on 101, or Crimestoppers, anonymously, on 0800 555111.