Buso Renkin (武装錬金, Busō Renkin?, lit. "Arms Alchemy") is a manga series written and drawn by Nobuhiro Watsuki, the creator of Rurouni Kenshin. Buso Renkin was serialized in Shueisha's shōnen magazine, Weekly Shōnen Jump from June 2003 to 2005, ending at 80 chapters. The individual chapters were collected by Shueisha into 10 tankōbon volumes which were released between January 5, 2004 and April 4, 2006. The series has been licensed by VIZ Media for US release and has been adapted into an anime by Xebec, which finished its run in Japan with a total of 26 episodes. There are also two drama CDs as well as a novel based on the series.

The story begins when high school student Kazuki Muto (武藤カズキ, Mutō Kazuki?) is killed one night saving a mysterious girl from a monster, only to wake up in his school dorm, believing it to have been a dream. However, he soon finds out that dream wasn't a dream at all when a giant serpentine monster attacks him and his sister. Tokiko Tsumura (津村斗貴子, Tsumura Tokiko?), the girl he saved, explains that the monster is a homunculus. Kazuki had been attacked and killed by it when rescuing Tokiko. However she, feeling responsible for him, revived him by placing a Kakugane medallion in his chest, serving as a replacement heart.

The Kakugane, as Tokiko explains, is an alchemical (alchemistic) device that, when activated, takes a certain form based on the Kakugane's user, forming a unique Buso Renkin. The Buso Renkin is the only thing that can destroy a homunculus monster. Using this, Kazuki creates his own Buso Renkin, taking the form of a giant lance, later named 'Sunlight Heart'. Along with Tokiko and her own Buso Renkin (the Valkyrie Skirt, an execution scythe), Kazuki joins the fight against the homunculi and their master.

Buso Renkin are weapons formed by the alchemically-created kakugane, based on the user's will to fight. The shape the Buso Renkin takes reflects the user's personality. Buso Renkin are the only weapons known to be able to kill a homunculus. Kakugane are developed in the process of creating the Philosopher's stone, the ultimate goal of the Alchemy Warriors around the planet. Kakugane will generate a weapon that reflects the user's battle style; a user will always have the same type of Buso Renkin regardless of the Kakugane number, though there will be cosmetic differences. Buso Renkin are activated by the fighting instinct of the user. This means that they can only be used by humans or human-type homunculi, since only humans have the instinct to use tools to defend themselves. Kakugane, in their normal state, can also strengthen a user's healing abilities. In total, there are only 100 Kakugane, from I (1) to C (100). The first three of were turned into Black Kakuganes and the second one was made into a White Kakugane afterwards. After the Victor Incident, Kakugane were scattered around the world; currently, there are 25 known Kakugane that are used in Japan.

In the world of Buso Renkin, there are two types of alchemy besides the historical one: buso renkin (arms alchemy) and homunculus. In this world, these are the only categories of alchemy which had succeeded, while the attempts to transform base metals into gold or achieve immortality had failed.