League of Legends (LoL), commonly referred to as League, is a 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 Windows and macOS.

In the game's main mode, Summoner's Rift, two teams of five players battle in player-versus-player combat. Each player controls a character, known as a "champion", with unique abilities and styles of play. During a match, champions become more powerful by collecting experience points, earning gold, and purchasing items to defeat the opposing team. Teams defend their base and win by pushing toward the enemy base and destroying a large structure within it, the "Nexus".

League of Legends has received generally favorable reviews, which have highlighted its accessibility, character designs, and production value. The game's long lifespan has resulted in a critical reappraisal, with reviews trending favorably; it is widely considered one of the greatest video games ever made. But rude and abusive in-game player behavior, criticized since the game's early days, 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, comic books, short stories, and the animated series Arcane. Its success has spawned several spin-off video games, including a mobile version, a digital collectible card game, and a turn-based role-playing game, among others. A massively multiplayer online role-playing game based on the property is in development.

League of Legends is the world's largest esport, with an international competitive scene consisting of multiple regional leagues which culminates in an 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 the cable television sports channel ESPN.

League of Legends is a multiplayer online battle arena (MOBA) game in which the player controls a character ("champion") with a set of unique abilities from an isometric perspective. During a match, champions gain levels by accruing experience points (XP) through killing enemies. Items can be acquired to increase champions' strength, and are bought with gold, which players accrue passively over time and earn actively by defeating the opposing team's minions, champions, or defensive structures. In the main game mode, Summoner's Rift, items are purchased through a shop menu available to players only when their champion is in the team's base. Each match is discrete; levels and items do not transfer from one match to another.

Summoner's Rift is the flagship game mode of League of Legends and the most prominent in professional-level play. The mode has a ranked competitive ladder; a matchmaking system determines a player's skill level and generates a starting rank from which they can climb. There are ten tiers; the least skilled are Iron, Bronze, and Silver, and the highest are Master, Grandmaster, and Challenger.

Two teams of five players compete to destroy the opposing team's "Nexus", which is guarded by the enemy champions and defensive structures known as "turrets". Each team's Nexus is in its base, where players start the game and reappear after death. Non-player characters known as minions are generated from each team's Nexus and advance toward the enemy base along three lanes guarded by turrets: top, middle, and bottom. Each team's base contains three "inhibitors", one behind the third tower from the center of each lane. Destroying an enemy team's inhibitor causes stronger allied minions to spawn in that lane and allows the attacking team to damage the enemy Nexus and the two turrets guarding it. The regions in between the lanes are known as the "jungle", which is inhabited by "monsters" that, like minions, respawn at regular intervals. Like minions, monsters provide gold and XP when killed. Another, more powerful class of monster resides within the river that separates each team's jungle. These monsters require multiple players to defeat and grant special abilities to their slayers' team. For example, teams can gain a powerful allied unit after killing the Rift Herald, permanent strength boosts by killing dragons, and stronger, more durable minions by killing Baron Nashor.

Summoner's Rift matches can last from 15 minutes to over an hour. The game does not dictate where players may go, but conventions have arisen: typically one player goes in the top lane, one in the middle lane, one in the jungle, and two in the bottom lane. Players in a lane kill minions to accumulate gold and XP (termed "farming") and try to prevent their opponent from doing the same. A fifth champion, known as a "jungler", farms the jungle monsters and, when powerful enough, assists their teammates in a lane.