0315 GMT July 23, 2019
Laurence Hart, the chief of the IOM mission to Afghanistan, said the trend was accelerated by the renewed US sanctions on Iran which sent the Iranian currency into freefall, according to AFP.
Hart, speaking in the Afghan city of Herat, said the trend was expected to continue.
"The reason why people are coming back is because of the reduced economic opportunities in the region...including Iran," Hart said.
The 773,125 voluntary returnees and deportees were 66 percent more than in 2017 and the trend is expected to continue, said Hart.
The 2018 figure was the highest since the IOM began systematically recording the volume of returnees to Afghanistan in 2012.
Many of the families of migrants are farmers who have been suffering through Afghanistan's worst drought in living memory, compounding the misery caused by 17 years of conflict and underscoring their reliance on the remittances.
"There were no jobs in Afghanistan so I had to go to Iran for work," said Mohammad Sarwar, 39, who worked as a laborer for four months before he was arrested by Iranian police and deported.
Abdul Hakim, 28, had just found a job in Iran after a month of searching when he was detained and kicked out. He faces an uncertain future as he tries to find a way to support his wife and three young children.
"The situation is very bad in Afghanistan," said Hakim, who comes from the northwestern province of Badghis, which has been hit hard by the drought.
Nearly half of the returnees – 358,065 – volunteered to come back to Afghanistan after watching their earnings shrivel up.
Meanwhile, just under 33,000 Afghans came back from Pakistan, where many have lived since fleeing the 1979 Soviet invasion.
An estimated 1.5-2 million "undocumented" Afghans are in Iran, the United Nations' refugee agency said in September, citing government estimates.
Another one million are registered as refugees.
For years, desperate Afghans have been paying smugglers $300-500 per person to cross the border with Iran in search of work to support struggling families.
Meanwhile, a group of 36 rejected Afghan asylum seekers deported from Germany landed in Kabul on January 8.
A total of 475 Afghan men have been sent back from Germany since 2016.
In July, an Afghan killed himself after being deported. The 23-year-old had lived in Germany for eight years before he was sent back to Kabul.