Claire Squires, a hairdresser from Leicestershire, was within one mile of the finish line, at Birdcage walk, near Buckingham Palace, when she collapsed to the ground.
She was immediately treated by medical teams, before being rushed by ambulance to hospital, where she was later pronounced dead.
A spokesman for the 26.2-mile run, said: “It is with regret that we have learnt of the death of a competitor.
“A 30-year-old woman collapsed at Birdcage Walk, and although immediate medical attention was provided to the casualty, the fatality was confirmed this afternoon.
“The organisers of the Virgin London Marathon would like to express their sincere condolences to the family and friends of the deceased.
“Our thoughts and deepest sympathies are with them at this difficult time.”
Squires died mid afternoon, however a precise time has not been given.
She becomes the tenth person to die in the London Marathon since it began in 1981.
The last competitor to die was a 22-year-old fitness instructor in 2007.
Most of the deaths had been caused by heart disease.
St John Ambulance said it treated 4,850 people at the marathon, while 30 people taken to hospital.
Some 37,000 people took part in yesterday’s event, which was won by Kenyan runners Wilson Kipsang and Mary Keitany.
They took first place in the men’s and women’s races respectively.
Kipsang crossed the finish line at two hours, four minutes and 44 seconds.
Keitany retained her London Marathon title at two hours, 18 minutes and 36 seconds, setting a new national record.
A host of celebrities also ran the race, including Nell McAndrew, James Cracknell, who both finished under three hours, plus newsreader Sophie Raworth, who finished in three hours 56 minutes.
Last year Raworth collapsed at the 23-mile mark.
British shadow chancellor Ed Balls came in at five hours and 33 minutes.
