Zarif will join a meeting of the foreign ministers from Britain, China, France, Germany, Russia and the United States at a meeting on Iran's contested nuclear program, Asthon told reporters.
The meeting, which is expected to be held Thursday, was scheduled to involve US Secretary of State John Kerry. The United States and Iran have no diplomatic relations.
Ashton said she had a "good and constructive" meeting at the United Nations with Zarif, a US-educated veteran diplomat named by newly elected President Hassan Rowhani.
"We have agreed that he and I will meet with our teams in Geneva in October," she said.
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Ashton played down expectations of a breakthrough, but voiced hope.
"I was struck by the energy and determination on the part of the minister," Ashton said.
But she said, "As you would appreciate, there is a huge amount of work to do."
US President Barack Obama has pledged to test whether Rowhani, who is considered moderate within the clerical regime, is serious about resolving Western and Israeli concerns about Iran's nuclear program.
Iran says its sensitive nuclear work is meant for peaceful purposes, but Western nations and Israel have voiced fear that the regime will seek a nuclear bomb.