The Hungarian produced a strong performance to take the title, holding off the USA's Haley Anderson, who finished second to claim silver. Italy's Martina Grimaldi took bronze.
Great Britain's Keri-anne Payne just missed out on a medal, finishing fourth.
Still only 26, Risztov competed in a full spectrum of events in London from the 4 x 100m Freestyle Relay through the 400m and 800m Freestyle as well as today's Marathon swim.
The nature of her victory was impressive, going out at the head of the field and never falling below third.
She said: 'It was a really tough race but I was prepared for the race and I'm very happy with the result.
'It is really hard to take in but in a few hours I will be much more happy because I'm really, really tired now.'