Iran had arrested nine local embassy staff, accusing them of instigating street protests over disputed presidential election results. It later released seven of them.
"Our ambassador was told that the eighth person would indeed be released today, that the papers have been signed and there would not be a court process or charges," Miliband told the BBC's Andrew Marr program. "That leaves one more in custody. All of our efforts are now directed at getting that person out."
Miliband said he had yet to speak to his Iranian counterpart about the status of the remaining detainee.
"I don't think you need to read anything into that. We've had two conversations over the course of the week," he said.
European Union countries summoned Iranian ambassadors on Friday to protest against the detention of the British embassy employees and warned of tougher steps next week if they are not freed.
(Reporting by Christina Fincher, editing by Tim Pearce)
