Rōshi (老師) (Japanese: "old teacher"; "old master") is a title in Zen Buddhism with different usages depending on sect and country. In Rinzai Zen, the term is reserved only for individuals who have received inka shōmei, meaning they have completed the entire kōan curriculum; this amounts to a total of fewer than 100 people at any given time. In Sōtō Zen and Sanbo Kyodan it is used more loosely. This is especially the case in the United States and Europe, where almost any teacher who has received dharma transmission might be called rōshi, or even use it to refer to themselves, a practice unheard of in Japan.

The Japanese rōshi is a translation of the more antiquated Chinese Laozi (Wade-Giles; Lao Tzu) meaning 'Old Master' and connoting the archetype of a wise old man. The modern Chinese 老師/老师 (Chinese pinyin: Lǎoshī) is a common word for teacher or professor without the religious or spiritual connotation of rōshi. Chinese Chán Buddhism (Zen is the Japanese transliteration of Chán) uses the semantically related title 師父/师父 or Mandarin shīfu (Cantonese "sifu"), literally "master father" or "father of masters", or 師傅/师傅, literally "master teacher" or "teacher of masters"; both pronounced "shīfu" in Mandarin) as an honorific title for the highest masters, but it also may be used in respectful address of monks and nuns generally.

Traditionally, the term rōshi has been applied as a respectful honorific to a significantly older Zen teacher considered to have matured in wisdom and to have attained a superior understanding and expression of the Dharma (Japanese: mujōdō no taigen) . Typically, a rōshi will have received dharma transmission (Jap: inka shōmei) many years ago and although often the abbot or spiritual director of a monastery may in fact be too old to carry these responsibilities.^{[citation needed]}

Despite this historical reality, it has come in some modern Zen schools to be applied as a general title for a teacher regardless of the age of the individual who receives it. This is especially true in the United States and Europe where it appears that some confusion has arisen where the word rōshi has been conflated with the term oshō, which is the generic term for a Soto Buddhist teacher who has received shiho and completed her or his basic training. Historically, the term rōshi will only be applied to an oshō after they have given many years of service as a teacher.

In some Rinzai organizations, a monastic is sometimes called rōshi after they have received inka shōmei, meaning they have completed kōan study and received Dharma transmission from their master;

In Rinzai Zen, it is relatively easy to say who is a roshi and who is not. Anyone who is authorized by another roshi (i.e. his teacher) is a roshi. This authorization (officially the "inka-shômei" document) is documented on a piece of paper, that is why it is also called colloquially "ichi-mai", that is "one sheet (of paper)". The transmission is totally vertical from teacher to student, no peer control is involved. That means that the Rinzai sect has no means to control who is made a roshi and who is not. In spite of that, the number of Rinzai roshis is usually less than 100 at any given time.

According to roshi Sokun Tsushimoto, the title of rōshi is equivalent to Zen master and shike:

'Roshi' is the title compatible with the most formal title ‘Shike’ who got officially authorized as a Dharma successor by authentic master.