Ackman, who runs hedge fund Pershing Square Capital Management, said on Twitter he also donated the shares to a donor-advised funds program and another non-profit he did not identify.
The donation is valued at $1.36 billion, calculated at Coupang's current share price of $51.4.
Ackman, who took home an estimated $1.4 billion last year after having delivered a 70.2% return, has previously supported a scholarship charity and other causes through his foundation.
Coupang, backed by SoftBank Group Corp, was valued at around $109 billion in its market debut on Thursday after it raised around $4.6 billion in the biggest U.S. initial public offering this year.
SoftBank also racked up a roughly $33 billion gain on paper through the public market debut of Coupang.
Founded in 2010 by Korean-American billionaire Bom Suk Kim, Coupang rose to prominence after launching its guaranteed same-day or next-day delivery service in the East Asian country.
"I was a very fortunate day one investor in @Coupang. As a result of its incredible success, 26.5m shares of its stock will benefit humanity as I gave all of the shares to @PershingSqFdn, a DAF, and another non-profit!," Ackman tweeted.
The billionaire investor has occasionally taken to Twitter, sending a message when he wants to get the word out to his 288,000 followers. But sometimes they are cryptic one-word answers like "yes" or "done" when he's asked something about his plans.
(Reporting by Noor Zainab Hussain in Bengaluru and Svea Herbst in Boston; Editing by Maju Samuel)
