The King of Fighters XI is a 2D fighting game produced by SNK Playmore originally released as a coin-operated arcade game for the Atomiswave platform in 2005. It is the eleventh installment in The King of Fighters series following The King of Fighters 2003.Set after the events of The King of Fighters 2003, the story focuses on a group known a "Those From the Past" who aim to obtain the power of the ancient demon Orochi. At the same time, a group of fighters seeks revenge against the protagonist Ash Crimson for stealing the powers of the clans in charge of sealing the creature. The game retains the gameplay elements from The King of Fighters 2003 involving fights between six fighters while at the same calling for assists It also provides new features allowing the player to perform simultaneous multiple special moves.

Development of The King of Fighters XI began after SNK Playmore finished making NeoGeo Battle Coliseum. The major changes to the gameplay and graphics were made possible thanks to the Atomiswave platform. Along with providing three new original playable characters, SNK Playmore decided to use characters from older fighting games including Buriki One and Kizuna Encounter to surprise fans.

During the previous series' installment, Mukai, a member of a mysterious group known as "Those from the Distant Land", stole the Orochi seal during the last tournament and broke it, causing confusion and destruction. Taking advantage of this situation, Ash Crimson attacked Chizuru Kagura in her weakened state and stole the Yata Mirror from her, draining her powers in the process. In a new King of Fighters tournament, new faces are handed invitations. These include Eiji Kisaragi (making his long-awaited comeback since his last appearance as a member of the Rival Team in The King of Fighters '95), who creates the Anti-Kyokugen Ryu Team and Kasumi Todoh, who also returns to The King of Fighters, and Malin. Oswald, an Irish card player who is an expert at using his playing cards to battle, joins Ash and Shen Woo on the Hero Team to find the whereabouts of a powerful new drug. Elisabeth Blanctorche, a young French woman who is acquainted with Ash, creates a Rival Team with Benimaru Nikaido and Duo Lon to investigate Ash's true intentions.

Duck King, a well-known South Town veteran fighter, joins Terry Bogard and Kim Kaphwan on the Fatal Fury Team as Joe Higashi had left to focus on his training, and Mai Shiranui had refused to help Terry out after he told her that he did not know where his young brother Andy was. Having recovered his powers lost during the NESTS Chronicles, Sie Kensou returns to the Psycho Soldiers Team with Athena Asamiya and newcomer Momoko, a cheerful capoeira-fighting girl.

Blue Mary forms a new Agent Team with returning characters Vanessa and Ramon. The female pirate B. Jenet, Gato and Tizoc form a new team, the MOTW Team. Due to an emotional trauma her character suffered at the end of The King of Fighters 2003, Leona leaves the Ikari Team to focus on regaining her strong will. Whip decides to leave the K′ Team to replace her. Former NESTS agent, Kula Diamond, joins K'. Maxima takes Whip's place on the K' team to help them investigate their latest threat. Since Chizuru is unable to help her teammates, Shingo Yabuki replaces her on the Sacred Treasures Team to help Kyo and Iori Yagami recover her powers from Ash.