Masayoshi Son, the billionaire founder of SoftBank Group Corp., made a huge personal bet on bitcoin just as prices for the digital currency peaked, losing more than $130 million when he sold out, according to people familiar with the matter.

Mr. Son, who launched the world’s biggest venture-capital fund on the strength of his long-term investing acumen, made the investment in late 2017 at the recommendation of a well-known bitcoin booster, whose investment firm SoftBank bought that year, the people said.

The...