SoftBank Group Corp shares jumped 13 percent on Thursday, a day after chief executive Masayoshi Son unveiled a $5.5 billion share buyback to prop up its share price, which he sees as undervalued.

SoftBank said after the market closed on Wednesday it would purchase up to 600 billion yen ($5.46 billion) of stock in its largest ever buyback, funded by the proceeds of its domestic telco's December IPO.

It also announced a 60 percent jump in third-quarter operating profit, lifted by rising valuations of its growing technology investments.

On Wednesday Son made a 90-minute presentation outlining SoftBank's investment strategy and what he sees as the chronic undervaluation of its shares.

While SoftBank's market value is currently around 9 trillion yen, when its stakes in telco SoftBank Corp, the Saudi-backed Vision Fund, chip designer Arm Holdings and others are taken into account, the company's holdings, net of debt, are worth 21 trillion yen, he said.

The buyback "puts a floor under SoftBank stock", Jefferies analyst Atul Goyal wrote in a note.

SoftBank shares hit a high in September but tumbled on concerns about its financial ties to Saudi Arabia following the murder of Saudi journalist Jamal Khashoggi.

The benchmark Nikkei 225 index was flat in early trading.