The companies announced the plan Thursday for a service that will use artificial intelligence and data analysis on a technology platform available for free to all taxi companies in Japan.
The service will roll out later this year, starting with a free trial run in Osaka, and expanding to Kyoto, Fukuoka, Okinawa and Tokyo, they said.
The move marks the arrival of a major Chinese mobile transportation platform in a nation that has resisted ride-sharing businesses and protected its traditional taxi companies.
The joint venture is mainly targeting the growing legions of Chinese tourists in Japan.
Didi President Jean Liu told reporters she has long been impressed with Japanese cab services and stressed the Didi service in Japan was "inclusive and collaborative," not disruptive, and will work as a plus for taxi drivers.
"I'm a big fan," she said of Japanese drivers, who she said were charming.
"We look forward to developing extensive collaboration with all industry players to assist in smart city programs in Japan and Asia."
It says its ride services, including car-sharing and food delivery, cover 30 million drivers using the platform and 550 million users.
SoftBank offers mobile and internet services. The SoftBank group also has IoT, robotics and energy businesses and is an investor in Grab, the leading ride-hailing company in Southeast Asia.
"I believe the joint venture can provide new value to both consumers and taxi companies in Japan," says SoftBank Chief Executive Ken Miyauchi.
Uber has set up shop in Japan but has not gotten far with car-sharing because of regulatory and bureaucratic hurdles. It is instead pushing UberEats, its food delivery business. Uber sold its China business to Didi, and holds a stake in Didi.