The New York Times had earlier reported that SoftBank was nearing a deal, but did not provide any financial details about the potential transaction.
ARM Holdings is the most valuable tech company listed in London.
It licenses its processor and graphics card technology to Apple for the iPhone, as well as Samsung, Huawei and others, giving it a near-universal presence in the mobile device market.
Any deal for ARM would come less than a month after Nikesh Arora, Son's heir apparent at SoftBank, quit the company.
SoftBank and ARM are yet to comment on the deal, but are expected to announce the acquisition later today. He said he wanted to develop Sprint and complete the transformation of SoftBank into an internet investment powerhouse. ARM designs have had little traction in data centers to date, and Intel dominates that market, but the large companies that operate those facilities have been avidly seeking alternatives.
Though it has warned on the staffing impact of Brexit, ARM Holdings' revenues are largely in dollars and it has a diverse portfolio.
Global media sources, including the Financial Times, are reporting that SoftBank has agreed to pay a massive premium to get ARM - reportedly £17 in cash for each share - a premium of more than 40 percent to Friday's close.