Ken Siegel is the managing partner of Morrison Foerster’s Tokyo office and the head of the office’s 50-attorney M&A team. Ken’s practice focuses on the representation of high-technology companies in acquisitions, joint ventures, and strategic alliances.
Ken has been consistently ranked as one of the leading U.S. attorneys in Japan. Among other things, he is the only attorney rated as a “Star Individual” for M&A in Japan (the level above Band 1) by Chambers Global, having been ranked at this level for more than a dozen years (2011–2024 editions). According to Chambers Asia-Pacific, Ken is “commercial and pragmatic, an incredible M&A attorney” who “deserves tremendous credit for his M&A practice” and has “outstanding acumen, experience and leadership.”
Ken received the Financial Times Innovative Lawyers Award in 2022, recognizing him as one of the most innovative practitioner in the Asia-Pacific region. In 2025, he and his team were recognized again by the Financial Times as Innovative Lawyers in Science and Technology for their groundbreaking AI work on Stargate, a joint venture developing advanced AI infrastructure in the United States.
He has also been recognized multiple times as in the American Lawyer – including as “Dealmaker of the Week” for his work on Hitachi’s $5 billion sale of its hard disk-drive business to Western Digital as well as for representing SoftBank in its $21.6 billion acquisition of Sprint, a transaction that was named the Global M&A Deal of the Year by The American Lawyer.
Ken received his B.A. magna cum laude from Amherst College. He studied Japanese language at Stanford University’s Inter-University Center for Japanese Language in Tokyo and at the Johns Hopkins University’s School of Advanced International Studies (SAIS), receiving a graduate degree in International Affairs. He received his J.D. degree from the University of Chicago Law School.
Ken’s recent notable awards include:
Most Innovative Legal Practitioner in Asia-Pacific (Financial Times Innovative Lawyers Award 2022)
SoftBank's acquisition of DigitalBridge
SoftBank on its $4 billion acquisition of DigitalBridge Group, Inc., a leading global alternative asset manager dedicated to investing in digital infrastructure.
SoftBank’s proposed acquisition of ABB’s robotics division
SoftBank on its proposed acquisition of Swiss-headquartered ABB’s robotics division at an enterprise value of $5.375 billion.
SoftBank's acquisition of Ampere
SoftBank in connection with its $6.5 billion acquisition of Ampere Computing, an American fabless semiconductor company developing processors for large-scale compute environments.
SoftBank’s proposed up to $40 billion investment in OpenAI
SoftBank, in connection with multiple investments in OpenAI, America’s premier artificial intelligence (AI) company and developer of the transformational large language models that are driving the AI revolution, including its proposed up to $40 billion investment in OpenAI.
SoftBank’s Stargate Joint Venture
SoftBank on The Stargate Project, including the formation of a new joint venture between SoftBank and OpenAI as lead partners, to invest billions of dollars in new AI infrastructure in the United States.
SoftBank’s joint venture with Tempus
SoftBank in connection with its investment in Tempus AI and the establishment of a Japan joint venture, SB TEMPUS Corp. with Tempus AI, Inc., to expand Tempus’s leading AI solutions for medical applications to Japan and Southeast Asia.
IPO of Arm Holdings plc
Arm Holdings plc in its $5.2 billion initial public offering and listing on Nasdaq, the largest global IPO of 2023.
SoftBank’s $280 million investment in Mapbox
SoftBank in its $280 million Series E investment in Mapbox, an AI-powered location services platform fueling navigation technology used by Toyota, General Motors and BMW.
Toshiba take-private
The board of Toshiba Corporation in its $15 billion take-private, the largest take-private transaction in Japanese history.
SoftBank’s sale of Fortress to Mubadala
SoftBank in its sale of 90.01% stake in Fortress, a global investment manager with over $45 billion in assets under management, to Mubadala.
SoftBank’s $375 million acquisition of Berkshire Grey
SoftBank in its $375 million acquisition of Berkshire Grey, Inc. (Nasdaq: BGRY), a robotic automation company, as well as interim financing through the completion of the merger.
Sprint’s $59 billion merger with T-Mobile
SoftBank, Sprint’s controlling shareholder, on the landmark merger of Sprint and T-Mobile US, Inc., with a total implied enterprise value for Sprint of $59 billion and an aggregate implied enterprise value for the combined company of $146 billion.
SoftBank’s £24.3 billion ($31.4 billion) acquisition of Arm Holdings
SoftBank in its £24.3 billion ($31.4 billion) acquisition of Arm Holdings, the largest-ever cross-border acquisition by a Japanese company (and the largest-ever cash acquisition of a UK company).
SoftBank’s $21.6 billion acquisition of Sprint
SoftBank in its $21.6 billion acquisition of a 78% interest in U.S. wireless operator Sprint. The transaction was the largest U.S. M&A transaction of 2012 and, when announced, the largest cross-border acquisition by a Japanese company (now second to SoftBank’s acquisition of Arm Holdings in 2016).
Toshiba’s $18 billion sale of Toshiba Memory Corporation
Toshiba Corporation in the $18 billion sale of its wholly owned subsidiary Toshiba Memory Corporation to a consortium led by Bain Capital, the largest private equity deal ever undertaken in Asia.
SoftBank’s $7.7 billion investment in Uber
SoftBank in its $7.7 billion primary and secondary investments as part of a $9.3 billion sale of stock by Uber. This is the largest-ever private secondary transaction in the technology space.
Hitachi’s $4.8 billion sale of Hitachi Global Storage Technologies to Western Digital
Hitachi in the $4.8 billion sale of its hard disk drive subsidiary, Hitachi Global Storage Technologies (HGST), to Western Digital. At the time of sale, HGST had over 40,000 employees worldwide and was one of the world’s leading HDD manufacturers.
Hitachi’s air conditioning joint venture with Johnson Controls
Hitachi and Hitachi Appliances on their formation of a global air conditioning joint venture with Johnson Controls, integrating the companies’ market-leading HVAC businesses. Johnson Controls obtained a 60 percent ownership stake in Hitachi Appliances’ more than $2.6 billion global air conditioning business.
TDK Corporation’s $3 billion RF filter joint venture with Qualcomm Incorporated
TDK in the formation of its $3 billion joint venture with Qualcomm to provide industry-leading radio frequency front-end (RFFE) modules and RF filters for mobile devices, drones, robotics, and automotive applications.