She collapsed at Birdcage Walk, near St James's Park, on the final stretch of the 26.2 mile course, the event's official website confirmed.
The woman was given medical attention at the scene but died this afternoon.
A statement on the marathon website said: "We would like to emphasise that our immediate concern is for the family of the deceased. Our thoughts and deepest sympathies are with them at this difficult time."
The statement added: "We will not be releasing any further details of this tragic incident until next of kin have been notified and formal identification has taken place. We would ask for your understanding in this matter."
The death occurred with the finishing line only one bend away.
Birdcage Walk borders St James's Park and is the last road that runners have to travel before reaching Buckingham Palace and turning onto The Mall, on which the finish line is located.Today's death was the tenth since the London Marathon began in 1981.
Five of the previous deaths were the result of heart disease in runners apparently unaware that they had a problem. Four of these were cases of severe coronary heart disease.
The last competitor to die was a 22-year-old fitness instructor in 2007.
