Sōtō Zen or the Sōtō school (曹洞宗 Sōtō-shū) is the largest of the three traditional sects of Zen in Japanese Buddhism (the others being Rinzai and Ōbaku). It is the Japanese line of the Chinese Cáodòng school, which was founded during the Tang Dynasty by Dòngshān Liánjiè. It emphasizes Shikantaza, meditation with no objects, anchors, or content. The meditator strives to be aware of the stream of thoughts, allowing them to arise and pass away without interference.
The Japanese brand of the sect was imported in the 13th century by Dōgen Zenji, who studied Cáodòng Buddhism (Chinese: 曹洞宗; pinyin: Cáodòng Zōng) abroad in China. Dōgen is remembered today as the co-patriarch of Sōtō Zen in Japan along with Keizan Jōkin.
With about 14,000 temples, Sōtō is one of the largest Japanese Buddhist organizations. Sōtō Zen is now also popular in the West, and in 1996 priests of the Sōtō Zen tradition formed the Soto Zen Buddhist Association based in North America.
The original Chinese version of Soto-shu, i.e. the Caodong-school (曹洞宗) was established by the Tang dynasty monk Dongshan Liangjie (洞山良价 Ja: Tōzan Ryōkai) in the 9th century.
One prevalent view is that the sect's name was originally formed by taking one character each from the names of Dongshan and his disciple Caoshan Benji(曹山本寂, Tōzan Ryōkai), and was originally called Dongcao sect (with the characters in transposed order). However, to paraphrase the Dongshan Yulu (《洞山語録》, "Record of the Dialogues of Dongshan"), the sect's name denotes 'colleagues (曹) of the teachings above the caves (洞)' who together follow the "black wind (teachings of Taoism?)" and admire the masters of various sects.
Perhaps more significantly for the Japanese brand of this sect, Dōgen among others advocated the reinterpretation that the "Cao" represents not Caoshan, but rather "Huineng of Caoxi temple" 曹渓慧能 (Sōkei Enō); zh:曹溪慧能). The branch that was founded by Caoshan died off, and Dōgen was a student of the other branch that survived in China.
A precursor to the sect is Shítóu Xīqiān (Ch. 石頭希遷, ca.700 – ca.790), the attributed author of the poem Sandokai, which formed the basis of Song of the Precious Mirror Samadhi of Dongshan Liangjie (Jp. Tōzan Ryōkai) and the teaching of the Five Ranks.