Folajimi “Jimi” Olubunmi-Adewole, 20, was on his way home from work at a central London restaurant when he and another man, Joaquin Garcia, entered the water at around midnight on Saturday April 24.
The Coastguard and the Metropolitan Police Marine Policing Unit rescued the woman and Mr Garcia, but could not locate Mr Olubunmi-Adewole.
On Tuesday, an inquest heard that an eyewitness saw the two men jump into the river near London Bridge in an attempt to save a woman, who could be heard shouting “Help me, help me, I can’t swim.”
The witness recalled seeing the pair run down steps to the river bank before partially undressing and jumping in.
Reading the evidence during the inquest opening at Inner London Coroner’s Court, assistant coroner Dr Julian Morris said: “They then saw one man jump in and swim out to the girl and appeared to be a good swimmer.
“The second male jumped some five to 10 seconds after and then began asking for assistance himself.”
Police arrived shortly afterwards and rescued Mr Garcia and the woman, but Mr Olubunmi-Adewole was found dead almost six hours later, the inquest heard.
The inquest was adjourned and a full inquest will be heard at a date to be fixed.
He was posthumously put forward for a Royal Humane Society award by City of London Police to honour his “memory and heroism” for his “bravery and selfless actions”.
An online petition to install a memorial plaque for Mr Olubunmi-Adewole in Postman’s Park, near St Paul’s Cathedral in central London, has reached almost 150,000 signatures.