Microsoft has agreed to invest about US$100 million ($152 million) in Uber Technologies at a valuation of around US$50 billion, a source says.
The deal was finalised last week, the source said.
Uber, which was previously valued at US$40 billion when it raised financing earlier this year, is benefiting from investors who view its car-booking technology as the future of transport.
The San Francisco-based company, led by chief executive Travis Kalanick, is using the cash to expand operations to cities around the world.
"We filed to authorise this new funding more than two months ago," said Kristin Carvell, a spokeswoman for Uber. "The filing is available to the public. We aren't commenting on additional speculation."
Tony Imperati, a spokesman for Microsoft, declined to comment.
The Wall Street Journal earlier reported that Uber closed a US$1 billion financing round at a valuation of about US$51 billion, and that Microsoft participated in the deal.
Microsoft last month sold a portion of its Bing mapping unit to Uber, which offered positions to about 100 Microsoft employees who work on image acquisition and data analysis as part of the mapping team for Bing, Microsoft's Web-search engine.
The two companies have also combined to integrate Uber into Microsoft's Cortana voice-controlled assistant so that it can hail an Uber ride for users as a scheduled meeting on their calendar approaches.
An investment, if it happens, would be a recognition that the two companies have worked together in the past and plan to remain partners in the future.
Uber, founded in 2009 by Kalanick and Garrett Camp, has expanded to more than 300 cities in at least 57 countries.
The service has disrupted established taxi and limousine companies, which have often responded with protests. Uber has also upset regulators, who have sued or banned the company from California to Brazil.