The deal, announced by Softbank's billionaire founder and chief Masayoshi Son and Sprint Chief Executive Dan Hesse at a packed news conference in Tokyo on Monday, gives Softbank entry into a U.S. market that is still growing, while Japan's market is stagnating.
It also gives Sprint the firepower to buy peers and build out its 4G network to compete in a market dominated by AT&T Inc and Verizon Wireless.
Shares in one of those potential targets, Clearwire Corp, surged nearly 16.3 percent to $2.70 in early trading.
Sprint owns 48 percent of Clearwire. While Softbank said no action was required, most analysts and investors see a Sprint-Clearwire tie-up as an inevitable consequence of the Softbank deal.
Analysts have long said the U.S. telecommunications industry needs to consolidate, but few looked to Japan as a catalyst. Some investors worried that Softbank is biting off more than it can chew.
But the 55-year-old Son, a rare risk-taker in Japan's often cautious business circles, is betting U.S. growth can offer relief from cut-throat competition in Japan's saturated mobile market. Combined, Softbank and Sprint will have 96 million users.
"It could be safe if you do nothing, and our challenge in the U.S. is not going to be easy at all. We must enter a new market, one with a different culture, and we must start again from zero after all we have built," Son told the news conference.
"But not taking this challenge will be a bigger risk."
Softbank said that as part of the deal, it will buy $3.1 billion of bonds convertible into Sprint stock at $5.25 a share, while about 55 percent of existing Sprint shares would be exchanged for $7.30 per share in cash, with the transactions to be completed by mid-2013.
Sprint added 1 percent to $5.80 early Monday after surging last week on first reports of a pending deal.
Hesse, who will stay on as Sprint CEO, said the Softbank investment would give Sprint opportunities it hadn't had since he joined the firm in late-2007, and enable the U.S. firm to play a bigger role in future market consolidation.
"This is pro-competitive and pro-consumer in the U.S. because it creates a stronger No. 3 ... it competes with the duopoly of AT&T and Verizon. When you look at what Softbank has accomplished in Japan with the No. 3 carrier, it's something we can learn from," he said.
At the press conference, Hesse noted it was the first time he had seen his long-time acquaintance Son with a tie on.
Softbank shares tumbled more than 8 percent on Monday before closing down 5.3 percent to their lowest finish in five months. The stock has lost more than one-fifth of its value - or $8.7 billion - since news first surfaced late last week about its interest in Sprint.
On Monday, credit rating agency Moody's said it was reviewing Softbank's ratings for a possible downgrade, but some analysts said Son's gamble might pay off in the end.
"It's the same (market) reaction as when Softbank said it was going to buy Vodafone a few years ago. Everyone came out and said it was far too expensive," Fumiyuki Nakanishi, general manager of investment and research at SMBC Friend Securities, said ahead of the announcement.
Softbank bought Vodafone's Japan unit for $15.5 billion in 2006.
"Son made a company worth 3 trillion yen, and now it will be worth 6 trillion yen. That's quite impressive, and I think investors will realize he's making the right decision down the road," said Nakanishi.
Four banks - Mizuho Financial Group Inc, Sumitomo Mitsui Financial Group, Mitsubishi UFJ Financial GroupM and Deutsche Bank - have approved loans totaling 1.65 trillion yen ($21.1 billion) to Softbank, three sources with direct knowledge of the matter told Reuters.
Sprint, which has lost money in its last 19 quarters, has net debt of about $15 billion, while Softbank has net debt of about $10 billion.
Brokers have warned the deal could leave Softbank with "unacceptably high" gearing - a ratio of debt to shareholder capital.
Standard & Poor's has warned the deal "may undermine Softbank's financial risk profile" and pressure its free operating cash flow for the next few years.
Reflecting the concerns, Softbank's 5-year credit default swap spreads - the cost of protecting its debt against default - widened to 267/327 basis points from around 160 basis points before the deal, and yields on its yen bonds have risen sharply.
NO CLEARWIRE OBLIGATION
Analysts have said the Softbank acquisition of 70 percent of Sprint for $20 billion would imply the No. 3 U.S. wireless company is worth about $28.6 billion, some two-thirds more than its market capitalization at Friday's close.
Sprint is going through a $7 billion upgrade of one of its networks, while closing its Nextel iDen network, which makes Softbank's capital especially useful.
An alliance with Sprint could also give Softbank leverage when dealing with Apple Inc, helping bolster its domestic position against KDDI Corp, which also offers the iPhone in Japan, and market leader NTT Docomo, which has yet to offer the Apple smartphone.
With Sprint in hand, Softbank may also look to acquire smaller U.S. carrier MetroPCS Communications Inc, Japanese media have reported.
Sprint has long had an interest in MetroPCS, which earlier this month agreed to merge with T-Mobile USA, part of Deutsche Telekom AG.
The Sprint deal takes outbound deals by Japanese firms to a record $75 billion this year, according to Thomson Reuters data, underscoring a strong appetite for overseas assets seemingly unaffected by signs of slowing global growth.
This is not the first Japanese foray into telecoms overseas. NTT Docomo racked up big losses after a string of failed investments in names like AT&T Wireless and Taiwan mobile operator KG Telecom in the late 1990s and early 2000s.
Raine Group LLC, a boutique merchant bank focused on the technology, media and telecoms sector, and Mizuho Securities were lead financial advisers to Softbank.
($1 = 78.3550 Japanese yen)
(Writing by Linda Sieg and Ben Berkowitz; Editing by Jeffrey Benkoe)