Dungeon Fighter Online, known in South Korea as Dungeon & Fighter, is a multiplayer beat 'em up action role-playing game, developed and published for personal computers by Neople, a South Korean subsidiary of Nexon, and originally published by Hangame in 2005. The game was originally released in South Korea as Dungeon & Fighter, then in Japan as Arad Senki, and then published in China by Tencent. A global version of Dungeon Fighter Online was released in May 2015.

According to Nexon, Dungeon Fighter Online has over 850 million registered users worldwide and over $22 billion in lifetime revenue as of June 2023, which would make it one of the highest-grossing entertainment media products of all time.

Dungeon Fighter Online is similar to 2D side-scrolling arcade hack and slash/beat 'em up games, such as Golden Axe or Double Dragon. Players traverse 2D screens while fighting hordes of monsters. There are a number of social aspects to Dungeon Fighter Online, including Guilds, PvP Arenas, and Party Play.

Skills can be designated upon an upper row of hotkeys, that can be further expanded by the decision of the player, who can choose to manually input the command to perform a certain skill; for example, a Blade Master can choose to press the assigned hotkey for the skill Draw Sword, but can also choose to perform its direct input. Directly inputting the skill (done by pressing the arrow keys in a certain sequence and then pressing the basic skill key) makes the skill cost less MP and lowers the cooldown of the skill (the time needed to wait to use the skill again) by a small amount. Skills are usually performed separately from normal combos, but some skills are "cancellable", meaning that those skills can be used in the middle of normal attacks.

Dungeon Fighter Online was developed by a South Korean company called Neople who previously only published a number of casual online games through their own game portal site. It was originally planned as a small game as the entire game was developed in five months based on the forecasted life expectancy, but the response was better than they expected so the budget was increased and the game was expanded. In terms of genre, Dungeon Fighter Online was initially planned as a fighting game similar to Tekken or The King of Fighters but this idea was scrapped as Neople felt that players wouldn't want to keep playing if there was a gap between them and their opponent's skill level. With this in mind, Neople looked for similar but different genres and ended up using belt-scrolling games such as Dynasty Wars and Cadillacs and Dinosaurs as an inspiration for making Dungeon Fighter Online an RPG.

Extensive testing took place before the premiere launch in Korea. Three closed beta periods were held between December 17–31, 2004, February 1–13, 2005, and June 28 – July 11, 2005. Neople accepted only 999 players per test and allowed only one hundred minutes of gameplay per day. Content was fine-tuned and updated daily throughout the test period based on testers feedback. After a short hiatus, open beta commenced on August 10, 2005 at 3 pm. By 11 pm, there were over 15,000 concurrent users.

Even though many games being released at the time were 3D, Neople decided to create Dungeon Fighter Online in 2D because they did not believe it affected gameplay, they did not feel a 3D game could capture the look and feel of the original illustrations of the characters, the ease of casual players getting into the game, and they had more experience with 2D games. Hi-res is not a likely path the game will take because director Yun Jong Kim's main focus is "efficiency".

Five years after the original release in Korea, Nexon America developed plans for an English version of Dungeon Fighter Online. The title was renamed from Dungeon & Fighter to Dungeon Fighter Online because of the awkwardness to say "Dungeon and Fighter".