TOKYO—SoftBank Group Corp. of Japan said Monday that it would separate its profitable but slow-growing domestic mobile business from its fast-paced but riskier overseas operations, which include a majority stake in U.S. mobile operator Sprint Corp.

Under the reorganization, SoftBank President Nikesh Arora, a former Google Inc. executive, will head the overseas management company, whose holdings also include a 32% share of Chinese e-commerce company Alibaba Group Holding Ltd. and a range of other Internet and technology companies....