President-elect Donald J. Trump’s first news conference since the election covered a wide range of topics, but it began with an eye-catching pledge: a $100 billion investment in the United States by SoftBank, a Japanese technology company.

Speaking from his Mar-a-Lago estate in Florida on Monday, Mr. Trump was accompanied by SoftBank’s chief executive, Masayoshi Son, perhaps best known in recent years for the speed and scope of his investments.

According to Mr. Trump, SoftBank will invest $100 billion in U.S. projects, intending to create 100,000 jobs. Trump transition officials said the investment will focus on technology and artificial intelligence over the next four years.

At the news conference, Mr. Trump immediately put the SoftBank chief on the spot, asking him to double the commitment. “He can actually afford to do that,” Mr. Trump said. Mr. Son was noncommittal but called the president-elect a great negotiator.

SoftBank doesn’t have $100 billion in cash on hand. The money is expected to come partly from its balance sheet but also from money raised alongside partners, according to a person familiar with SoftBank’s plans.

Mr. Son, 67, started SoftBank in Tokyo in 1981 and has built it into one of the largest technology investors in the world. His firm has been known for its high-profile wins and losses, and Mr. Son — commonly known as Masa — has been known to grab headlines with eye-popping pronouncements, fund-raising and investments.