The IEA said the release had been backed unanimously by 32 member countries, including Australia, in the sixth such move it has made since its creation in the 1970s.
US President Donald Trump said the decision to release oil from strategic reserves would substantially reduce oil prices.
"I'm pleased to report that earlier today the International Energy Agency agreed to co-ordinate the release of a record 400 million barrels of oil from various national petroleum reserves around the world, which will substantially reduce oil prices as we end this threat to America and the world," Trump said.
The release is aimed at preventing a further rise in oil prices on fears that Iranian attacks will continue to block Middle East oil exports from reaching markets.
"The oil market challenges we are facing are unprecedented in scale, therefore I am very glad that IEA member countries have responded with an emergency collective action of unprecedented size," IEA executive director Fatih Birol said.
The Paris-based IEA made its comments as French President Emmanuel Macron chaired a meeting of G7 leaders to discuss the issue.
"The emergency stocks will be made available to the market over a time frame that is appropriate to the national circumstances of each member country," the IEA said, adding this would be "supplemented by additional emergency measures by some countries".
Earlier Trump had said in a local media interview his nation would contribute "a little bit" from the US Strategic Petroleum Reserve.
"I filled it up once, and I'll fill it up again, but right now, we'll reduce it a little bit, and that brings the prices down. We have to get rid of the evil. There's great evil taking place in Iran, as you know," he said in an interview with Cincinnati's Local 12.
Oil prices gained nearly five per cent on Wednesday, US time, as further attacks on ships in the Strait of Hormuz worsened supply-disruption fears, and analysts said the release of reserves was inadequate to ease those concerns.
Analysts have said the pace of daily IEA stock releases would matter as much as if not more than the overall size.
If 100 million barrels were released over the next month, the daily pace will amount to about 3.3 million barrels per day - a fraction of the current disruption of about 20 million barrels per day, with the Strait of Hormuz between Iran and Oman effectively blocked.
In 2022, IEA member countries released 182.7 million barrels of oil and oil products in two stages, which was then the largest in IEA history, when Russia launched its full-scale invasion of Ukraine. The US was by far the top contributor to that release.
"Pressure came mainly from the US government which wants this release," a European Union diplomat had said, speaking before the IEA statement.
G7 member Japan said it planned to release about 80 million barrels from its private and state oil reserves as its contribution.
"Rather than wait for formal IEA approval of a coordinated international reserve release, Japan will act first to ease global energy market supply and demand, releasing reserves as early as the 16th of this month," Prime Minister Sanae Takaichi said in a statement.
India, also a major importer of Middle East oil, welcomed the IEA's announcement.
"India stands ready to take appropriate measures, as necessary, to support global market stability in alignment with the efforts of the International Energy Agency," India's ministry of petroleum and natural gas said in a statement.
Several countries co-ordinate their strategic oil stockpiles through the IEA, which was formed in 1974 after the oil crisis.
IEA members hold emergency stockpiles of more than 1.2 billion barrels, with another 600 million in industry stocks held under government obligation.
