The Japanese majority owner of Sprint(S) - Get Report said on Monday it agreed to offer 1,700 pence per share for the Cambridge, England-based target, or 43% more than Friday's 1,189 pence per share closing price.
The takeover of ARM will give the Japanese Internet and telecoms investor a company whose semiconductor know-how is used in 95% of feature and smartphones globally and increased clout to compete in the growing market for connected devices.
The purchase marks a dramatic departure from SoftBank's recent asset-sale drive, which has included a June deal worth $7.3 billion to sell its majority stake in Finnish games maker Supercell to Tencent and co-investors and the sale of $8.9 billion worth of Alibaba(BABA) - Get Report securities, bringing down its holding to about 28%.
"We have long admired ARM as a world renowned and highly respected technology company that is by some distance the market-leader in its ld," said SoftBank CEO and founder Masayoshi Son on Monday. " ARM will be an excellent strategic fit within the SoftBank group as we invest to capture the very significant opportunities provided by the 'Internet of Things'. "
Shares in the target surged above the value of the offer in early trading in London, suggesting some investors expect a sweetened bid. They were recently up almost 45% at 1,721 pence.
SoftBank has about $80 billion of debt and had been working to reduce it, though the surprise news last month that one-time CEO-in-waiting Nikesh Arora would step down as chief operating officer, allowing Son more time at the helm, may have changed those calculations.
The U.K's June 23 vote to leave the European Union - another surprise - may also have had an impact.
The pound has fallen 11% against the dollar since the vote, making shares of London-listed companies cheaper.
SoftBank said it will finance the purchase with ¥1 trillion of debt from Mizuho Bank.
ARM is the global leader in so-called licensable central processing unit cores for mobile and low power applications and derives all of its revenue in dollars.
Peel Hunt analysts expect the company to report 2016 sales of £1.135 billion, up from £968.2 million and pretax profit of £576.1 million, up from £511.5 million. They said the weaker pound will lift earnings per share by 8% in 2016, and 12% in the following two years.
In keeping with the typical playbook of Japanese acquirers, SoftBank has said it will keep the ARM organization, including senior management, intact, keep its Cambridge, England headquarters and "at least double" its headcount in the U.K. over the next five years.
The pledges also speak to concerns about foreign takeovers of key U.K. companies that Prime Minister Theresa May raised last week before she succeeded David Cameron at the helm.
Japan stock exchanges were closed for a public holiday on Monday. SoftBank closed on Friday at ¥6,007.0, assigning a ¥7.2 trillion ($68.2 billion) market value to the whole group.
The acquirer had revenue in the fiscal year ended March of ¥9.15 billion and Ebit of ¥1.04 billion.
It plans to conduct the purchase through a so-called scheme of arrangement, which requires the approval of 75% of ARM's shareholders by value and a majority by number.