The attacker, who was caught assaulting the woman in Holloway, north London, was jailed for six years by Blackfriars Crown Court.
The case went ahead under the assumed name Aurilien Cheyroux, as the authorities could not confirm his true identity, other than him being French.
The judge ruled he should be deported once his sentence was served.
Passers-by saw "Cheyroux" partially-dressed and struggling with a woman on Caledonian Road in the early hours of 1 March, the court heard.
He ran off, but was caught by witnesses and held until police arrived.
The woman told the witnesses she was uninjured and left the scene before police arrived.
The attacker told police he was a French national and gave the name used in the trial.
But after taking finger prints the French authorities said the information was incorrect. They were unable to establish a true identity.
Det Const Kip Malek said: "This is a highly unusual case where we have been unable to trace the victim and the man who has been convicted is under an assumed name.
"Despite this we have been able to get a conviction and that is due to the brave members of the public who witnessed the attack and took it upon themselves to pursue this man and contact us."
Cheyroux was found guilty of attempted rape but had previously pleading guilty to sexual assault.
Det Insp Neil Smithson added: "I am still keen to ascertain his identity. Equally, if you were the victim of this offence please contact us."
A man has been jailed for attempted rape despite police not knowing who he is or finding his victim.