Lloyd Gardner, 22, of Ottery St Mary, Devon, contacted police in 2006 after seeing CCTV footage of a man wanted in connection with the rape in Exeter.
He spotted two women he knew on the film - and they led police to rapist Jakub Tomczak.
Mr Gardner said he did not deserve the reward and hoped the cash would help the woman rebuild her life.
He said: "I was told I was entitled to a reward because I gave police the lead they needed as I recognised the two girls and through them detectives were able to track the man down.
"It was a difficult decision to make because it is a lot of money and it would have been very helpful but I didn't feel like a deserved it at all.
"With the state that the girl was in after the incident, I really felt that it would go towards making her life a lot better over the next couple of years."
The 48-year-old woman who was attacked in July 2006 was found naked and unconscious in Exeter.
She had suffered a skull fracture and brain damage in the attack and her injuries were so severe she was left with no memory of what had happened and now uses a wheelchair.
Mr Gardner added: "I know that it was the right thing to do. I would have probably spent it on an expensive car or something which doesn't seem right so I am happy with my decision."
Jakub Tomczak, a Polish national, was found guilty of rape and causing grievous bodily harm at Exeter Crown Court in January last year and was given two life sentences to be served in Poland.
The 25-year-old, who was a law student at the time, had been on a night out in the city, and his movements were tracked by CCTV camera.
After the attack, he returned to Poland where, four months later, he voluntarily gave a DNA sample to local police at the request of UK officers.
The sample was analysed in the UK, where it matched the DNA profile taken from semen found on the victim.
He was brought back to the UK to face trial under a European arrest warrant, before being repatriated to Poland after his conviction.
He had denied the attack, saying he had gone home after becoming separated from friends.