League of Legends (LoL), commonly referred to as League, is a 2009 multiplayer online battle arena video game developed and published by Riot Games. Inspired by Defense of the Ancients, a custom map for Warcraft III, Riot's founders sought to develop a stand-alone game in the same genre. Since its release in October 2009, League has been free-to-play and is monetized through purchasable character customization. The game is available for Microsoft Windows and macOS.

In the game, two teams of five players battle in player-versus-player combat, each team occupying and defending their half of the map. Each of the ten players controls a character, known as a "champion", with unique abilities and differing styles of play. During a match, champions become more powerful by collecting experience points, earning gold, and purchasing items to defeat the opposing team. In League's main mode, Summoner's Rift, a team wins by pushing through to the enemy base and destroying their "Nexus", a large structure located within.

League of Legends has received generally positive reviews; critics highlighted its accessibility, character designs, and production value. The game's long lifespan has resulted in a critical reappraisal, with reviews trending positively; the negative and abusive in-game behavior of its players, criticized since early in the game's lifetime, persists despite Riot's attempts to fix the problem. In 2019, League regularly peaked at eight million concurrent players, and its popularity has led to tie-ins such as music videos, comic books, short stories, and the animated series Arcane. Its success has spawned several spin-off video games, including League of Legends: Wild Rift, a mobile version, Legends of Runeterra, a digital collectible card game and Ruined King: A League of Legends Story, a turn-based role-playing game, among others. A massively multiplayer online role-playing game based on the property is in development.

Considered one of the greatest video games ever made, League of Legends is the world's largest esport, the game has an international competitive scene consisting of multiple regional leagues. They culminate in the annual League of Legends World Championship. The 2019 event registered over 100 million unique viewers, peaking at a concurrent viewership of 44 million during the finals. Domestic and international events have been broadcast on livestreaming websites such as Twitch, YouTube, Bilibili, and on cable television sports channel ESPN.