The open-top bus tours will cross the Thames four times, at Waterloo Bridge, Westminster Bridge, London Bridge and Tower Bridge.
"By teaming up with Stagecoach London to deliver our megasightseeing.com services we're bringing our brilliant value fares to a whole new audience," said Megabus managing director Mark Venables.
"Around 19 million tourists visit London every year and this is the perfect sightseeing choice for those coming to this great city – a great value, non-stop tour of the capital's most famous sights, from a brilliant top-deck vantage point.
"We look forward to welcoming people from around the globe on to our services."
Paul Lynch, managing director of Stagecoach London, added: "We are so proud to be launching this new product.
"With our superb value tickets this is not only a fantastic service for tourists coming to London but for people living or working in or around London who might have a couple of hours to spare, or friends and relatives visiting from out of town.
"With our great value tickets a family can enjoy a day out for a great price."
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