Baguazhen (Chinese: 八卦陣; pinyin: bāguàzhèn; lit. 'eight trigrams (military) formation') or Bagua Formation, also known as Jiujun (lit. 'Nine armies') or Jiǔgōng bāguàzhèn (九宮八卦陣; 'Nine palaces', 'eight trigrams formation') or Bazhen (八陣; 'eight formations') is a military formation originating from China, based on the principles of Bagua or Qimen Dunjia. The formation has been used throughout Chinese history and attributed to many famous military figures, most notably to Zhuge Liang. In Song dynasty sources, the bazhen is also alternatively known as jiujunzhen (九軍陣; 'nine-army formation').

Generally, the bazhen is a mass military formation, where the army are divided into eight units deployed at cardinal and ordinal directions, often surrounding a central commanding unit. The exact composition and the names of the regiments in the baguazhen varies in different sources, but the key feature of the formation is its defensive capability and adaptability, by quickly mobilizing and shifting unit ranks on the field, to cover for each unit's weaknesses.

In Questions and Replies between Tang Taizong and Li Weigong, the formation is described as being composed of large, medium and small units, with each unit being composed of, and able to divide into, smaller battalions. Roughly, the large unit is composed of eight medium units, and the medium units can be split into six small squad units, making a total of 64 squad units per formation. Some sources also call for an additional shock cavalry numbering in 24 squads, bringing the number to a total of 88. The central unit would be composed of small number of elite troops, protected by large number of movable infantry. Each unit situated in each direction could respond to any attack and function as each others' defence and reserve, by shifting their positions and surrounding the attacker from all sides, or moving its weaker units behind the stronger units. Other sources such as Taiping Yulan also records the use of war wagons, such as the "deer-antler wagon" (鹿角車), to obstruct the enemy while providing added protection and quick transport for the soldiers.

The regiments are also separated into zheng (正)-troops or regular troops, and qi (奇)-troops, or special troops, respectively. The zheng and qi are based on Sun Tzu's tactic of applying direct, ordinary force to engage in battle and for indirect, inordinary force to secure victory. ("凡戰者，以正合，以奇勝").

The names of the eight units vary. In the dialog between Li Jing and Li Shimin, it is said that the regiments are named Tian (天, 'heaven'), Di (地, 'earth'), Feng (風, 'wind'), Yun (雲, 'cloud'), Long (龍, 'dragon'), Hu (虎, 'tiger'), Niao (鳥, 'bird') and She (蛇, 'snake'), with heaven, earth, wind and cloud symbolising flags and banners, and dragon, tiger, bird and snake corresponded to squad units. These units are further categorized into four zheng-units and four qi-units, respectively.

In Wujing Zongyao, the eight formations are the 'sky-formation' (天陣), 'earth-formation' (地陣), 'wind-formation' (風陣), 'cloud-formation' (雲陣), 'tiger wing-formation' (虎翼陣), 'snake coil-formation' (蛇盤陣), 'flying dragon-formation'(飛龍陣), and 'bird flying-formation' (鳥翔陣), with each having their assigned position within the formation, with the animal formations supporting the elemental formations, while lesser elemental forms support the greater.

Wang Yingling of the Song dynasty records in the book Xiaoxue Ganzhu (小学绀珠) that the names of the eight formations are Dongdang (洞當), Zhonghuang (中黃), Longteng (龍騰), Niaofei (鳥飛), Zhechong (折衝), Huyi (虎翼), Woji (握機), and Lianheng (連衡).

Other sources claim the eight are named after the eight gates of Qimen Dunjia, called Xiu (休), Sheng (生), Shang (伤), Du (杜), Jing (景), Si (死), Jing (惊), and Kai (开).