Witnesses said the woman was being dragged out by the water’s strong current when quick-thinking members of public threw rope to pull her back towards the bank.
She desperately tried to keep her head above the water before police and a lifeboat arrived to drag her onto dry land.
Australian Brad Hancock was passing by on Wednesday evening when he noticed a group of people gathered near the Oxo Tower.
He told the Standard: “I was walking on my way to Waterloo as I do every night. And there was all this commotion in front of ITV studios.
“At first I thought was a stunt but I fast realised a women was in the water. I don’t know how she got in there.
“People were trying to throw things in the water because the current was dragging her further out.
“She was struggling to keep her head above the water so I ran along the promenade and found a red box with a rope and safety device.
Massive thanks to @MPSonthewater and @TowerRNLI for getting to the scene so quickly. Susanna well done for holding on to the rope until our amazing emergency services arrived. Hope you make a full recovery. #Londoners you make me proud! Hope in humanity restored!— Brad Hancock (@BDHancock) February 21, 2018
“I opened it and threw it in and she luckily caught it and I tried to calm her until the RNLI and police came.
“She sank underwater a few times but I told her to wrap the rope around her body and I wouldn’t let go.
“Someone also found a life ring and threw it in but because she was completely clothed, she struggled to hold it.”
The 36-year-old, who works as head of digital at a leading asset management firm, said the Met Police and Tower RNLI arrived within minutes.
He said: “Once the water police arrived with the RNLI they managed to pull her out of the water.
“I’m just glad we all managed to think quickly and I found that rope. It saved her life.
“Everyone did what they could to help the poor women until the real heroes arrived. I did what any person would have done to help save a life.”
Mr Hancock, who has lived in London for 12 years, added: “I was just doing my duty as a Londoner and as all the Londoners did to help someone in distress.
“It restored my faith that people do care for each other. And I’m happy I played a very small part in that.”
He said the lifeboat then sped off on the water with the woman.
A spokesman for Tower RNLI said: “We were called at 5.35pm to a person in the water near the Oxo Tower.
“We recovered a female and took them to a place of safety before handing them into the care of London Ambulance Service.”
The condition of the woman could not be confirmed.