“I sincerely apologize for not being able to continue my service and all the shortcomings and faults during my term,” he wrote in his post.
It remains unclear why Zarif would offer his resignation, or if it has been accepted.
The deputy spokesman for Iran's foreign ministry, Abbas Mousavi, said Zarif had resigned, according to the official Islamic Republic News Agency. But a spokesperson for the Iran Mission to the United Nations told ABC News that Iran's president, Hassan Rouhani, had not accepted the resignation.
Rouhani's chief of staff, Mahmoud Vaezi, wrote in a Twitter post that Rouhani had yet to accept Zarif's resignation.
Zarif’s Instagram post came on the same day Syrian President Bashar al-Assad paid a surprise visit to Iran's capital, Tehran, and met with Rouhani and Iranian Supreme Leader Ayatollah Ali Khomeini.
Zarif did not appear in any of the photos of these meetings, a sign that could possibly signify disagreements between top Iranian officials.
According to Iran's semi-official Entekhab News Agency, Zarif responded to a reporter's text message asking whether he had resigned, saying, "After the photos of today's visits, Javad Zarif has no credit in the world as a foreign minister."
ABC News' Marcus Wilford contributed reporting from London.