Reda al-Basha, 27, the DMI's Tehran news correspondent for about one year, was not on assignment when he went missing near his home on December 28, DMI Managing Director Ahmad Abdulla al-Shaikh said in a statement.
Earlier on Sunday, Tehran General Prosecutor Abbas Jafari Dolatabadi told the semi-official Fars News Agency the journalist had been freed.
Basha was arrested the day after eight people were killed in clashes between security forces and supporters of opposition leader Mirhossein Mousavi on Ashura, a day of ritual Shi'ite mourning.
It was the worst violence since that which accompanied protests immediately after the disputed presidential election in June.
Opposition protesters say the vote was rigged in favour of re-elected President Mahmoud Ahmadinejad, but the government denies this.
The opposition website Rahesabz said on Wednesday that more than 180 people, including 17 journalists, 10 Mousavi aides and some members of the outlawed Baha'i faith, were arrested in the aftermath of the December 27 Ashura protests.
A European diplomat also arrested on December 27 was held for 24 hours. Dolatabadi said on Sunday the diplomat was Swedish.
"This diplomat at the Swedish embassy in Tehran was arrested by security forces during the unrest on Ashura, but he was released after his identity was verified," Dolatabadi told Fars.
He said on Friday that five detainees whom Iran plans to try in connection with last month's anti-government protests were members of the People's Mujahideen Organisation of Iran (PMOI), an exile group opposed to Iran’s Islamic system of government.
Separately, an opposition website on Sunday said about 30 "mourning mothers", women whose children disappeared or were killed during post-election unrest, were arrested in a Tehran park on Saturday and taken to a detention centre in the capital.
The mothers gather in Tehran's Laleh park every Saturday, the Kaleme website said. The report could not be independently verified.
Keep up with the latest business news from the region with the Emirates Business 24|7 daily newsletter. To subscribe to the newsletter, please click here.