Just hours before the Oscar ceremony in Los Angeles, where the film is in the running for the Best Foreign Language Film award, the iconic central London landmark will transform into a giant open-air cinema for the 4.30pm (1630 GMT) showing.
"It will be a great opportunity to showcase how London is an international hub of creativity and a global beacon for openness and diversity," said London mayor Sadiq Khan, who will speak before the screening.
Farhadi thanked Khan and the cinema community "for this generous initiative", adding he appreciated "this invaluable show of solidarity."
"The gathering of the audience around 'The Salesman' in this famous London square is symbolic of unity against the division and separation of people," he told the Guardian.
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Farhadi announced last month that he would not attend the ceremony even if the US government gave him special permission to travel despite coming from Iran, one of the countries on Trump's controversial list.
Around 10,000 spectators are expected for the screening, which will be the British premiere of the film, according to the mayor's office.
British director Mike Leigh is also expected to speak before the screening, followed by a mini concert by The Orchestra of Syrian Musicians.
Around fifty film personalities, including Ridley Scott, Kiera Knightley, Terry Gilliam, Glenn Close, and Julie Christie, signed a letter asking that the film be screened in front of the US Embassy in London.
"We wish to hold an event in solidarity with Mr Farhadi himself, but crucially, with the many thousands of innocent people who will now be negatively impacted and harmed by a policy of outright discrimination such as this," they wrote.