The new City Hall-funded campaign is due to launch in September and will target audiences across Europe, the United States and Asia.
It aims to promote the capital by celebrating London’s “rich heritage, world-class experiences, culture, creativity and role as a centre for innovation and trade”.
Sir Sadiq said: “We are facing a relentless and unprecedented attack of lies and hatred from those wanting to damage our capital’s standing and our hugely important tourism industry.
“Disinformation about London has become a truly global scourge.
“It’s a money-making industry pushing lies about our capital and preying on people’s fears around the world, so we must fight back on a global scale.
“That’s why we’re creating this major new campaign to challenge these false narratives and demonstrate once again why there’s no better place in the world than London.”
Reform UK’s candidate for mayor of London, Laila Cunningham, said: “If the mayor of London wants to encourage more tourism, he should deal with the crime he has allowed to spiral out of control instead of suggesting that victims of crime are liars.”