In the first quarter, Buffett's Berkshire Hathaway bought an astounding 75 million shares of Apple. That adds to the 165.3 million shares Berkshire already owned at the end of 2017.
In a statement to CNBC, Apple CEO Tim Cook said the company is "thrilled to have Warren and Berkshire as a major investor."
"On a personal level, I've always greatly admired Warren and have always been grateful for his insight and advice," Cook said.
As of Thursday's closing price of $176.89 per share, Berkshire's Apple stake was worth about $42.5 billion. Berkshire is now the third largest Apple shareholder, behind Vanguard and BlackRock.
Apple also hit an all-time high Friday morning after Buffett's revelation.
"It is an unbelievable company," Buffett says. "If you look at Apple, I think it earns almost twice as much as the second most profitable company in the United States."
Buffett revealed the additional purchases of Apple in an interview Thursday evening just ahead of the Berkshire Hathaway annual shareholders meeting in Omaha, where 40,000 Berkshire shareholders will arrive this weekend.
He explained that Berkshire's quarterly earnings report, which will be released on Saturday, includes information on Berkshire's 10 largest holdings, and that astute readers of that report would be able to detect Berkshire's larger interest in Apple.
Apple shares were under pressure ahead of Tuesday's after-the-bell earnings report in the face of a gloom-and-doom outlook about iPhone sales, and speculation the tech giant was winding down iPhone X production. But Apple ended calming those fears with a stronger-than-expected quarterly earnings report.
"The idea that you're going to spend loads of time trying to guess how many iPhone X ... are going to be sold in a 3 month period totally misses the point," Buffett says. "It's like worrying about the number of BlackBerrys 10 years ago."