Investors were spooked when the Japanese lender's Vision Fund reported another heavy loss for the third quarter running.
The investment arm made losses of $9.9 billion over the three months to Sept. 30.
SoftBank shares were down 13% Monday, and headed to their biggest one-day loss in more than two and a half years.
Analysts blamed the slide in part on a pullback from a steep rally.
Last week, SoftBank had announced completion of its plan to spend just under $2.9 billion buying back its own shares.
Such moves had helped the stock gain more than 40% since October.
Market specialists also pointed to worries about a broader weakness in tech stocks.
That hurts SoftBank as the lender bets heavily on the growth of high-tech companies.
The company as a whole reported its first quarterly profit in three quarters.
It was helped by cutting some of its stake in China's Alibaba.
