Motamedi received the award at Thursday's closing ceremony, held on November 12, and used his acceptance speech to express pride in the solidarity of the Iranian people during difficult times.
"I am proud to be one of the Iranian people, who, under the most difficult circumstances, have stood together and stood by our country," he said, addressing filmmakers and other guests from across Eurasia and beyond.
The award was presented by Margarita Simonyan, editor-in-chief of the Rossiya Segodnya international media group, which includes the RT television network.
The Best Screenplay prize carries a monetary award of $250,000.
The film follows a middle-aged man caring for his mother, who is in a coma, while hoping she will regain consciousness. Motamedi said he hoped to release the film in Iran around Nowruz, the Persian New Year, in the coming months.
Iranian cinema was also represented in the competition by A Lunch Promise, directed by Majid Majidi. Keyvan Moghadam, the production designer for The Daughter of Lady Fairy, was among the three finalists in the production design category.
The second edition of the awards brought together 41 films from 19 countries, with winners announced in 12 categories.
The festival's top honor, Best Film, went to the Russian production August, directed by Nikita Vysotsky and Ilya Lebedev. The film received a trophy and a 10-carat Yakut diamond valued at $1 million.
The awards ceremony began with a video message from Russian President Vladimir Putin.
