The authorities in Gibraltar also released the captain of the Grace 1, who is Indian, and three officers who had been arrested and bailed without charge pending the outcome of the investigation, the Gibraltar Chronicle reported.
The decision to release the ship was taken hours after the US launched a separate last-minute legal move to detain the vessel and its cargo, which is owned by the National Iranian Oil Company and is worth $140m.
In India, Minister of State for External Affairs, V. Muraleedharan tweeted: "Spoke to our High Commission @HCI_London on VLCC Grace 1. They confirmed all 24 Indian crew aboard VLCC Grace 1 have been released by Gibraltar authorities and are free to return to India."
The Chief Minister of Gibraltar Fabian Picardo, in a statement, said the decision to release the ship follows a letter received from Iran on August 13 assuring that, if released, the "destination of Grace 1 would not be an entity that is subject to European Union sanctions".
"The Grace 1 is therefore now released from detention under the Sanctions Act by operation of law", he said.
Earlier, at a short hearing on Thursday, the Gibraltar Supreme Court was told the Government had received a request from the US Department of Justice for mutual legal assistance to commence new proceedings in a bid to seize the ship.
The basis for the request was not revealed in court but must now be assessed by the Ministry for Justice and the Government Law Offices in Gibraltar, which will then decide how to proceed.
In court, Chief Justice Anthony Dudley said there was no formal US application currently before him to seize the vessel.
On hearing that the Gibraltar Government was no longer seeking to detain the tanker under sanctions legislation, Mr Justice Dudley added: "She is no longer a specified ship...and no longer subject to detention."
Chief Minister Picardo said Gibraltar's mutual legal assistance authorities would now make "an objective, legal determination" of the US request for legal proceedings, the Gibraltar Chronicle said.
But as of Thursday afternoon, barring any fresh application before the court, he was clear the Grace 1 was free to sail from Gibraltar.
The Gibraltar Foreign and Commonwealth Office said Iran must abide by the assurances it has provided that the Grace 1 will not proceed to Syria.
The FCO said there is no comparison or linkage between what it called Iran's "unacceptable and illegal" seizure of commercial vessels in the Strait of Hormuz, and Gibraltar's enforcement of EU Syria sanctions.
Meanwhile, Iranian Foreign Minister Javad Zarif tweeted that the Iranian embassy in London, in its note to the UK Foreign and Commonwealth Office, "Protested the illegal detention of Grace1; -Repeated public statements about destination; & -Made it crystal clear that EU sanctions do NOT apply to Iran" a position shared by most Europeans.
"Nothing changes the fact: detention was 100% unlawful," he posted.
A three-member team from the Indian High Commission had met the 24 Ind on board 'Grace 1' on July 24 and told them the government will take the necessary steps for their release and repatriation.
The Iranian super tanker Grace 1, said to be carrying 2.1 million barrels of light crude oil, was seized on July 4 in the Strait of Gibraltar as it was suspected of carrying oil to war-torn Syria in violation of EU sanctions.
On July 19, Gibraltar's Supreme Court extended the period of detention of Grace 1, and set a new hearing for August 15, the government of the British Overseas Territory said in a statement.
( With inputs from IANS )