The Touhou Project (Japanese: 東方Project, Hepburn: Tōhō Purojekuto; sometimes written in Japanese as 東方プロジェクト), also known simply as Touhou (東方; meaning "Eastern" or "Oriental"), is a bullet hell shoot 'em up video game series created by independent Japanese doujin soft developer Team Shanghai Alice. The team's sole member, Jun'ya "ZUN" Ōta, has independently developed programming, graphics, writing, and music for the series, publishing 20 mainline games and 13 spin-offs since 1997. ZUN has also produced related print works and music albums, and collaborated with doujin developer Twilight Frontier on seven of the official spin-offs, six of which are fighting games.

The first five games were developed for the Japanese PC-98 computer, with the first, Highly Responsive to Prayers, released in August 1997; the series' signature danmaku (弾幕; lit. 'bullet curtain') mechanics were introduced in the second game, Story of Eastern Wonderland (also 1997). The release of Embodiment of Scarlet Devil in August 2002 marked a shift to Microsoft Windows. Numerous sequels followed, including several spin-offs departing from the traditional shoot 'em up format.

The Touhou Project is set in Gensokyo, a preternatural land sealed from the outside world and primarily inhabited by humans and yōkai, legendary creatures from Japanese folklore that are personified as bishōjo in an anthropomorphic moe style. Reimu Hakurei, the miko of the Hakurei Shrine and the main character of the series, is often tasked with resolving supernatural "incidents" caused in and around Gensokyo; she is joined by Marisa Kirisame after the events of the second game.

The Touhou Project has become more particularly notable as a prominent source of Japanese doujin content, with the series spawning a vast amount of fan-made works such as artwork, music, print works, video games, and Internet memes. Because of this, it has gained a large cult following outside of Japan. The popularity of the series and its derivative works has been attributed in part to the few restrictions placed by ZUN on the use of his content. Unofficial works are frequently sold at fan conventions, including Comiket, where the franchise has frequently held the record for circle participation, and the official convention Reitaisai, where trial versions of the official games are typically distributed prior to release.

Jun'ya "ZUN" Ōta, who was then a mathematics student at Tokyo Denki University working under the name "ZUN Soft", developed the first five Touhou Project games for the PC-98 computer, utilizing the platform's 16-bit color graphics and 6-channel FM synthesis audio. The games were published by Amusement Makers, a student game development club that Ōta was a member of.

After the release of Mystic Square, ZUN graduated from university and the series became inactive for four years. During this time, he worked at Taito as a game developer and also composed music for various games created by members of Amusement Makers. He left the group in 2001 to focus on game development for Microsoft Windows, forming the one-man dōjin circle Team Shanghai Alice and self-publishing all subsequent games. According to ZUN, the Windows games represent a "clean slate" for the series canon, albeit with many carry-overs and references from the PC-98 era. Games numbered with decimals are spin-offs from the main series that vary in genre.

In the games, the player's bullet power increases on a linear scale as the player collects power-ups dropped by enemies, and eventually maxes out. The player can also collect "point" icons to earn extra lives; the amount needed grows exponentially as the player's score rises. The player can enter "focus mode" by holding the shift key by default, which slows down the player's movement, makes their hitbox visible (from Perfect Cherry Blossom onwards), and generally focuses the player's attack to make it more powerful. The graze counter (missing from Mountain of Faith, Story of Eastern Wonderland, and Unconnected Marketeers) tracks how many bullets entered the character sprite but avoided the hitbox, rewarding the player with a score bonus for taking risks.

The player can use a "bomb" or "spell card", similar to the "bomb" in many other shooting games. Although the player has a limited number at any given time, losing a life replenishes the current number of bombs up to a certain amount. With some exceptions, use of a bomb will make the user temporarily invulnerable, clear many of the bullets, and cause heavy damage to any enemies on screen. The overall effect the bomb has varies by character and by game. The player can use one during a short period after being hit by a bullet (called a "deathbomb") to avoid loss of a life. The amount of time the player has available to deathbomb is usually around 0.3 seconds (8 frames). Bosses have attack phases, which are also referred to as spell cards, but with bosses the term applies to a prolonged pattern of movements and shots that last until the player depletes the boss' health by a certain amount or the time runs out.