Photo of SoftBank logo Headquartered in Tokyo, SoftBank will be establishing a new fund platform.

TOKYO and NEW YORK CITY—In a surprise move, Tokyo-based SoftBank Group Corp. said Tuesday it would merge with Fortress Investment Group LLC, the alternative asset manager which has been among the most active in commercial real estate. Expected to close in the second half of this year, the deal calls for SoftBank to acquire Fortress for approximately $3.3 billion in cash.