By Phred Dvorak

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 pair of resignations brings to an end around a decade and a half during which the two powerful entrepreneurs -- Mr. Son from Japan and Mr. Ma from China -- cemented their close relationship by serving as directors of each others' companies. Mr. Son has been on Alibaba's board since 2005, five years after SoftBank took a major stake in the then-small Chinese online retailer. Mr. Ma came on SoftBank's board a few years later, in 2007.

Mr. Son said he was stepping down at his own request, and that he was "matching" the day of his resignation with Mr. Ma's. He said that the parting of ways was smooth, and that he hadn't had a fight with Alibaba.

Mr. Son famously decided to invest $20 million in Alibaba in 2000, after a five-minute chat with Mr. Ma. That investment has become SoftBank's, and Mr. Son's, most successful, and its large holding in Alibaba currently supports much of the Japanese company's credit rating, share price and funding activities.

SoftBank has at times sold some of that stake to raise money, most recently this year, when it entered into derivative transactions to sell $11.5 billion worth of Alibaba shares to fund share buybacks as well as bolster its balance sheet.

Write to Phred Dvorak at phred.dvorak@wsj.com