The 5.5-metre (20ft) model, two-thirds the size of the original, was made using 3D digital scanning technology and Italian marble.
It was created by the Institute for Digital Archaeology (IDA), a joint venture between Harvard University, the University of Oxford and Dubai’s Museum of the Future.
"Palmyra was the perfect example in its day of east/west cooperation and the arch symbolizes that as much today as it did back then," said IDA director Roger Michel on Tuesday.
"No one should have the power to delete such monuments from our historical record."
Unveiling the monument, Mayor Johnson said: "this is an arch of triumph and in many ways a triumph of technology and determination.
"We`re here in a spirit of defiance, defiance of the barbarians who destroyed the original ... as they have destroyed so many other relics in Syria and the Middle East."
The United Nations cultural organization UNESCO calls Palmyra one of the most important cultural centers of the ancient world.
"From the 1st to the 2nd century, the art and architecture of Palmyra, standing at the crossroads of several civilizations, married Greco-Roman techniques with local traditions and Persian influences," it adds on its website.
The replica arch will stand in London for three days before going to Dubai, New York and ultimately back to Palmyra itself which was recaptured at the end of March after having been under the control of IS for some 10 months.
Recreation of destroyed Palmyra arch unveiled in London
23 April 2016
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A meticulous recreation of the 1,800-year-old Arch of Triumph in Palmyra, Syria, destroyed by Islamic State militants last year, has been erected in Trafalgar Square in what London Mayor Boris Johnson called an act of defiance.
