SoftBank's shares soared 6.2 per cent, helping push Japan's Nikkei up 0.7 per cent, Reuters reported.
In the lobby of Trump Tower in Manhattan, Son said his telecommunications and tech investment company would create jobs by investing in US startups.
Trump, naturally, took credit for the investment, later tweeting that the investment wouldn't have been made if he hadn't won the election.
Masa said he would never do this had we (Trump) not won the election!— Donald J. Trump (@realDonaldTrump) December 6, 2016
Son, whose company owns 82 per cent of US telecoms giant Sprint Corp said he expected more "deregulation" under Trump, causing speculation that Sprint could bring back merger talks with T-Mobile that dissolved under the pressure of US regulators.
Sprint's shares reached their highest level in two and a half years after Son's announcement before falling slightly to close up 1.5 per cent.
Trump and Son did not give a timeline for the investment.
Softbank gave UK tech a boost in July this year when it revealed plans to buy Arm Holdings for £24bn.