OpenAI did not disclose the size or terms of the offering, and said a timeline has not yet been determined.
"It may be a while because there are things we want to do that are likely easier as a private company," it said in a statement.
Reuters had reported that the AI giant is targeting a valuation of up to $1 trillion in a stock market debut that could come as early as September.
At that valuation, OpenAI would set the stage for a trio of trillion-dollar valuation companies debuting rapidly and is seen as the most consequential test of investor appetite for high-growth technology stocks in the recent decade.
Elon Musk's SpaceX was the first off the block, filing for an IPO that would rank as the largest in history if completed, with the company pursuing a $US75 billion offering at a $US1.75 trillion valuation.
On prediction markets, where traders wager on the outcome of future events, most participants had expected OpenAI to file for an IPO before Anthropic.
The IPOs from Anthropic and OpenAI would crystallise a transformative period for the technology industry and global markets, with artificial intelligence rapidly emerging as the defining investment theme of the decade.
OpenAI said earlier this year that it was raising $US110 billion at an $US840 billion valuation from a roster of heavyweight backers including SoftBank, Amazon and Nvidia.
At the time, it also disclosed that ChatGPT had more than 900 million weekly active users and over 50 million consumer subscribers.
The IPO filing follows OpenAI renegotiating its partnership with Microsoft, one of its earliest investors, that allowed the AI pioneer to forge new partnerships with firms such as Amazon.com and Alphabet's Google unit.
The Windows maker's early investment, totalling $US13 billion since 2019, helped pave the way for OpenAI's rapid rise and powered growth at the software major's Azure cloud-computing business.
In March, OpenAI said it was generating $US2 billion in monthly revenue and growing roughly four times faster than companies that defined the internet and mobile eras, including Alphabet and Meta.
That compares with about $US1 billion in quarterly revenue at the end of 2024.
OpenAI was founded in 2015 as a research-focused nonprofit, but created a for-profit arm four years later to help fund the soaring costs of developing artificial intelligence systems.
Its unusual structure, which gave the nonprofit control over the for-profit entity, came under intense scrutiny in late 2023 when CEO Sam Altman was briefly ousted before returning days later after employees revolted.
In December 2024, OpenAI unveiled plans to overhaul its structure by creating a public benefit corporation, saying the move would help it raise far more capital while easing restrictions imposed by its nonprofit parent.
OpenAI's overhaul quickly became controversial after sharp criticism from its early backer, billionaire Elon Musk, who later sued OpenAI and accused Altman and other executives of turning the nonprofit into a vehicle for private enrichment.
A US jury in May ruled against Musk in his lawsuit, finding the AI company not liable to the world's richest person for having allegedly strayed from its original mission to benefit humanity.
The unanimous verdict removed a key overhang on the IPO.
