Winny (also known as WinNY) is a Japanese peer-to-peer (P2P) file-sharing program developed by Isamu Kaneko, a research assistant at the University of Tokyo in 2002. Like Freenet, a user must add an encrypted node list in order to connect to other nodes on the network. Users choose three cluster words which symbolize their interests, and then Winny connects to other nodes which share these cluster words, downloading and storing encrypted data from cache of these neighbors in a distributed data store. If users want a particular file, they set up triggers (keywords), and Winny will download files marked by these triggers. The encryption was meant to provide anonymity, but Winny also included bulletin boards where users would announce uploads, and the IP address of posters could be discovered through these boards. While Freenet was implemented in Java, Winny was implemented as a Windows C++ application.

The software takes its name from WinMX, where the M and the X are each advanced one letter in the Latin alphabet, to N and Y. Netagent published a survey in June 2018 suggesting that Winny was still the most popular p2p network in Japan ahead of Perfect Dark (P2P) and Share (P2P) with approximately 45,000 nodes connecting each day over Golden Week. The number of nodes on Winny appears to be holding steady compared with 2015.

Kaneko first announced Winny on the Download Software board of the 2channel (2ch for short) Japanese bulletin board site. Since 2channel users often refer to anonymous users by their post numbers, Kaneko came to be known as "Mr. 47" ("47-Shi", or 47氏 in Japanese), or just "47".

After Winny's development stopped, a new peer-to-peer application, Share, was developed to be a successor.