As part of the deal, it will ship smartphones and tablets loaded with Microsoft software, including Office and Skype, to buyers in India and China.
No financial details of the partnership were disclosed.
"Xiaomi and Microsoft announced today a deepening of our partnership... The companies' new collaboration also includes a cross-license and patent transfer agreement," Xiaomi said in an emailed response.
Microsoft sold Xiaomi nearly 1,500 patents that read on a variety of technologies from its worldwide portfolio, including wireless communications, video, cloud and multimedia, it added.
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Xiaomi will start pre-loading Microsoft Office and Skype on its phones starting September 2016, initially in China, with other markets including India to follow.
This will include Mi 5, Mi Max, Mi 4S, Redmi Note 3 and Redmi 3 devices from Xiaomi.
"Microsoft Corp and Xiaomi Inc have expanded their global partnership to provide innovative user experiences on mobile devices... As a result, tens of millions of consumers and business customers in China, India and around the world will have new ways to work, collaborate and communicate," Microsoft said in a statement.