He announced the deal after meeting with SoftBank CEO Masayoshi Son, a Japanese billionaire and technology investor, at Trump Tower in New York. On Twitter, Trump claimed that Son said he "would never do this" if Trump had not won the presidential election.
But the $50 billion will come from a previously announced $100 billion international investment fund set up with Saudi Arabia, according to Dow Jones.
The pair did not give details on what specific investments SoftBank would make, though Son suggested the Japanese company would target the technology sector.
Sprint and T-Mobile shares popped after the news, but gave up most of those gains. Son has previously expressed interest in combining the companies, hoping to take on larger rivals, but he has faced regulatory resistance.
The paperwork from Trump's meeting with SoftBank also featured the logo of Foxconn, also known as Hon Hai, a supplier for Apple's iPhones, Son noted. It was unclear what role the company would play in the deal.
Trump said after the election he told Apple CEO Tim Cook that he hopes Apple will "build a big plant in the United States, or many big plants in the United States."
"I do believe that some manufacturing may come back to the United States, primarily because of the fact that president-elect Donald Trump is going to twist Apple's arm," Colin Gillis, an analyst at BGC Partners, told CNBC's "Power Lunch" on Tuesday. "And there's been reports that Foxconn is reviewing being able to assemble plants, and it would be a good thing."
Still, when President Barack Obama tried to pressure Apple into moving manufacturing jobs back to the U.S, co-founder Steve Jobs told Obama "those jobs aren't coming back," according a 2012 New York Timesreport.
"I do think the reality is, it would only be a sliver, a percentage of the phones that were made that were assembled in the United States," Gillis said. "We're talking, some de minimis amount merely to try to check the box."
CNBC has reached out to Apple and Foxconn for comment.