Billionaire investor Carl Icahn took to Twitter yesterday to unveil his latest investment: Apple.

“We currently have a large position in Apple,” he tweeted a little after 2 p.m. “We believe the company to be extremely undervalued. Spoke to [Apple CEO] Tim Cook today. More to come.”

Shares of Apple immediately rose about 4 percent, and closed the day up almost 5 percent, at $489.86.

“We really like the company a lot,” Icahn told The Post. “I think they can afford and should do a $150 billion buyback now.”

Icahn’s comments came just hours after Oracle CEO Larry Ellison said in a TV interview that he thinks Apple’s best days are behind it.

“I think Larry Ellison is a very smart guy, but a lot of smart guys were also negative on Netflix when we started buying buying it below 60 less than a year ago,” Icahn said. It closed yesterday at $259.19.