TOKYO—SoftBank Group Corp. reported its first quarterly net loss in a year and a half after China’s stricter regulations on technology companies caused a sharp stock selloff and weighed on SoftBank’s investment portfolio.

The Japanese conglomerate, best known for its $100 billion Vision Fund and one of the world’s leading funders of Chinese startups, posted a loss of ¥398 billion, equivalent to $3.5 billion, for the quarter ended Sept. 30.

“It...

TOKYO— SoftBank Group Corp. reported its first quarterly net loss in a year and a half after China’s stricter regulations on technology companies caused a sharp stock selloff and weighed on SoftBank’s investment portfolio.

The Japanese conglomerate, best known for its $100 billion Vision Fund and one of the world’s leading funders of Chinese startups, posted a loss of ¥398 billion, equivalent to $3.5 billion, for the quarter ended Sept. 30.

“It is a time of severe trials for China’s high-tech stocks,” said SoftBank’s chief executive, Masayoshi Son. “We are right in the middle of a storm.” He added that he remained optimistic about his company’s investments.

Mr. Son said the company did much worse in the quarter by his preferred performance measure, net asset value. SoftBank Group’s net assets fell by more than $50 billion to stand at the equivalent of $184 billion, he said.

The leading reason, he said, was a fall in the share price of Chinese e-commerce giant Alibaba Group Holding Ltd. , long SoftBank Group’s most valuable holding. Alibaba fell 35% in the July-September quarter.

Mr. Son said SoftBank would buy back ¥1 trillion, equivalent to about $8.8 billion, in its own shares, with a goal of completing the buyback within a year.

In addition to its China issues, SoftBank said a big factor in the loss was a fall in the share price of South Korean e-commerce giant Coupang Inc., one of the companies in which the Vision Fund has invested.

The results marked SoftBank’s first loss since the January-March quarter of 2020, when it recorded its worst result—nearly a $9 billion full-year loss—amid the pandemic. In the April-June 2021 quarter, the company recorded a net profit of more than $6.7 billion.

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In the latest quarter, SoftBank recorded an investment loss at the Vision Fund equivalent to about $10.3 billion. That compared with a $2.5 billion gain in the April-June quarter.

SoftBank’s shares closed Monday down 0.8% at ¥6,161 before the release of the results. Shares have fallen more than 40% since the year’s peak reached in March. Mr. Son called SoftBank’s stock undervalued.

Beijing has been tightening regulations on its largest tech companies this year, weighing on the value of those companies’ shares and indirectly hitting SoftBank as well.

Shares of Chinese ride-hailing leader Didi Global Inc.,

in which the Vision Fund holds a stake, fell 45% in the July-September quarter.

On Nov. 1, a privacy law that will curb data collection by technology companies went into effect in China.

Write to Megumi Fujikawa at megumi.fujikawa@wsj.com