The California company's "Everyone's Uber" service went live in the morning in the western city of Fukuoka, the firm's research partner, Kyushu University, said.
In the pilot programme, users who need a ride pin their location and request a driver through the app downloaded to a smartphone or tablet.
Uber is already active as a paid-for taxi service in Tokyo, but during the data-gathering pilot, passengers in Fukuoka will not need to pay.
The city's Kyushu University will analyse data collected as part of research into mobility and transportation, Uber said.
Fukuoka city is one of Japan's designated special economic zones, where companies and universities are encouraged to launch renewable energy and energy-saving pilot programmes.
San Francisco-based Uber has grown into one of the world's largest startups, valued at some $40 billion, with operations in more than 200 cities in 54 countries around the world.
The company does not employ drivers or own its own vehicles, but instead uses independent contractors with their own cars.
Passengers, who have registered their credit card with the company in advance, request a ride through the app, which then matches up potential drivers.
Real-time mapping using GPS shows customers exactly where their taxi is.
Charging is usually done by time, and billed automatically to the passenger's credit card. Uber then makes payment to the driver.
Uber's rapid spread has generated friction with existing taxi operations, and posed challenges for regulators.
Taxi drivers in dozens of cities have staged protests against Uber, and regulators in many cities have sought to shut it down.
Safety concerns are also rising.
An Indian woman, who alleges an Uber driver raped her in New Delhi, has sued the online taxi service in a US court, accusing it of failing to ensure passenger safety.Catch the latest from the Consumer Electronics Show on Gadgets 360, at our CES 2026 hub.