The male victim was attacked in the early hours of Friday morning.
East of England Ambulance Service were called to the alleged incident in Chelmsford, Essex at around 4.55am yesterday.
They told police that a man had claimed to them he had been raped.
Officers reviewed CCTV from the scene and made enquiries.
The 25-year-old was arrested on suspicion of rape and remains in custody for questioning.
A police cordon was put in place behind the Bar&Beyond nightclub in the centre of Chelmsford.
Charlie Blake, who works nearby, told Essex Live: “This morning I saw police forensics in white suits taking pictures of the street and the alley.”
In a statement, Essex Police said: “We were called by the ambulance service at around 4.55am today, Friday, August 9, after a man told paramedics he had been raped in an alleyway off Baddow Road, Chelmsford.
“We made enquiries at the scene and reviewed CCTV before arresting a 25-year-old man at around 6.40am on suspicion of rape. He remains in custody for questioning.
“A man told paramedics he had been raped in an alleyway”
Essex Police statement
“Anyone with information or dashcam footage is asked to call our Major Crime Team on 101 quoting crime reference 42/127440/19 or Crimestoppers anonymously on 0800 555 111.”
A spokesman for Bar&Beyond said: “As this is an ongoing police investigation, all we can confirm is that the alleged incident is not linked to our venue.”
It came just hours after two men were stabbed in Chelmsford on Thursday evening near The Meadows Shopping Centre.
The double stabbing was just 100m from the site of the later rape, but there is no indication the two incidents are linked.