TOKYO—SoftBank Group Corp. Chief Executive Masayoshi Son said Thursday that he is stepping down from the board of Chinese e-commerce giant Alibaba Group Holding Ltd., effective the same day.

Mr. Son’s announcement, which he made at the end of SoftBank’s annual shareholder meeting, follows the resignation—also effective Thursday but announced last month—of Alibaba co-founder Jack Ma from SoftBank’s board.

The…

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