Average rent prices for one room were a whopping £1,182 in Westminster, nearly £500 more than in Woolwich, where city dwellers pay £693.
Revealing the prices of different locations along the route, WeRoom.com's map showed homes surrounding the final few miles of the race as the most expensive.
Areas such as Crossharbour and Deptford were also priced at less than £800.
CEO of WeRoom Thomas Villeneuve said: "Our data shows how expensive renting a room in London can be.
"The areas surrounding the first miles on the marathon route are certainly more affordable than those along the home stretch near Westminster and Waterloo, but paying almost £700 a month to rent a room in the capital can still be a costly affair for many Londoners."
The race takes place on April 24, at 10am, with spectators gathering along the route to support the runners. If you're going along, be sure to get the best viewing spots across the city.
You can see the full 26.2-mile marathon route here.