Mendeley is a desktop and web program produced by Elsevier for managing and sharing research papers, discovering research data and collaborating online. It combines Mendeley Desktop, a PDF and reference management application available for Windows, macOS and Linux. It also provides Mendeley for Android and iOS, with Mendeley Web, an online social network for researchers.

Mendeley requires the user to store all basic citation data on its serversstoring copies of documents is at the user's discretion. Upon registration, Mendeley provides the user with 2 GB of free web storage space, which is upgradeable at a cost.

Mendeley, named after the biologist Gregor Mendel and chemist Dmitri Mendeleyev, was founded in November 2007 by three German PhD students and is based in London. The first public beta version was released in August 2008. The company's investors include some people previously involved with, Skype, and Warner Music Group, as well as academics from Cambridge and Johns Hopkins University.

Mendeley was purchased by the Elsevier publishing company in 2013. The deal price was speculated to be 50 million (US$65 million), following earlier speculation that it was between $69 million and $100 million. The sale led to debate on scientific networks and in the media interested in Open Access, and upset members of the scientific community who felt that the program's acquisition by publishing giant Elsevier, known for implementing restrictive publishing practices, the high prices of their journals (see The Cost of Knowledge), and publicly supporting the SOPA bill, was antithetical to the open sharing model of Mendeley. David Dobbs, in The New Yorker, suggested Elsevier's reasons for buying Mendeley could have been to acquire its user data and/or to "destroy or coopt an open-science icon that threatens its business model."

In 2012, Mendeley was one of the repositories for green Open Access recommended by Peter Suber. The recommendation was revoked in 2013 after Elsevier bought Mendeley.

In 2018, an update to Mendeley resulted in some users losing PDFs and annotations stored in their accounts. After a number of weeks, Elsevier announced a potential fix for this problem.

In 2019, an update to Mendeley, encrypting the backup database, resulted in users being unable to use third-party tools to access the literature database. This made, for example, a transfer to other citation managers preserving the folder structure impossible with the latest version of Mendeley.