The services were blocked when there were widespread protests against former president Mahmoud Ahmadinejad.
According to Reuters, the unfreezing of the service has not been officially announced, but foreign press living in Tehran for the New York Times and the Washington Post both said on Twitter that they could access the service freely.
Jillian York, the director for international freedom of expression at the Electronic Frontier Foundation, has received multiple reports from citizens using several different Iranian internet service providers, confirming that the bans appeared to have been lifted.
Other sites which had been blocked are also suddenly unblocked.
Iranian authorities blacked out Facebook and Twitter in the summer of 2009, when Ahmadinejad's disputed re-election victory sparked massive protests. Social media was used to organise demonstrations.
New president, the moderate cleric Hassan Rouhani, has indicated that he will adopt a much different tack from that of its hardline predecessor.