HANGZHOU, China—Alibaba Group’s sprawling campus has collegial workspaces, laid-back coffee bars and, on the landscaped grounds, a police outpost.

Employees use the office to report suspected crimes to the police, according to people familiar with the operation. Police also use it to request data from Alibaba for their own investigations, these people said, tapping into the trove of information the tech giant collects through its e-commerce and financial-payment networks.

In...