The incident is alleged to have taken place near Weekday Cross.
Detective Sergeant Hannah Frame, of Nottinghamshire Police, said: “Police were called shortly after midnight on Saturday 23 April to Weekday Cross, Nottingham city centre, following allegations of a sexual assault against a young woman.
“A 16-year-old boy was arrested a short time later on suspicion of rape and has been released on police bail. Investigations continue but we believe this was an isolated incident.
“Nottinghamshire Police treats sexual offences extremely seriously and our specially-trained public protection officers are highly skilled in conducting investigations into this type of crime – with a focus on sensitivity, compassion and safeguarding for victims.”