Officers were called at around 3.15am yesterday to reports a woman had been assaulted by a man in Lexden Road, near the Essex County Hospital.
Detectives are keen to speak to a man who was in the area carrying a blue carrier bag.
The area has been cordoned off while investigations take place
DI Stuart Truss said: “This is likely to have been a busy road at that time of night, with people walking out of Colchester town centre.
“I would appeal for anyone who may have seen a man and woman hanging around Essex County Hospital at that time to contact police.
“The victim is being cared for by specialist officers”.
Anyone with any information is asked to contact Essex Police on 101 or Crimestoppers anonymously on 0800 555 111.