Siming (Chinese: 司命; pinyin: Sīmìng) refers to a Chinese male deity or deified functionary of that title who makes fine adjustments to human fate, with various English translations (such as, the Master of Fate, Controller of Fate, Deified Judge of Life, Arbiter of Fate, Director of Allotted Life Spans, and Director of Destinies). Siming is both an abstract deity (or title thereof) and a celestial asterism.

Siming, as Director of Destinies, has the bureaucratic function of human lifespan allocation. Siming seems to have roots in the shamanic traditions, then later to have somewhat assimilated with the Kitchen God, as in the Daoist case of the Three Worms, in which Siming becomes a deity to whom home household activities are periodically reported,

As an asterism, or apparent stellar constellation, Siming is associated with the Wenchang Wang star pattern, near the Big Dipper, in what is more or less Aquarius.

Sometimes the term Siming is qualified by 大 (da, meaning "big" or "greater") or by 少 (shao, meaning "small" or "lesser").

The Siming deity has the bureaucratic function of human lifespan allocation. In bureaucratic terms, Si (司) is a common term, meaning "in charge of", "a person or department which is in charge of something", often translated as "secretary". Commonly, in real life or in imagined bureaucracies, there were Chief and Assistant Secretaries (Dasi and Shaosi). Ming (命) is a complicated word with a long folk and technical history, basically meaning "life" or "the balance of fate or destiny", personified as Siming. Often, a deified entity such as Siming receives increased sanctity over time, signified by additional official titles.

As a deity Siming takes his, her, or their place in a complex cosmological system of Chinese religion and mythology. Over time, this system became a visualization of a complex cosmology including the elaboration of a heavenly bureaucracy, somewhat parallel to the earthly bureaucracy of the Chinese state, and invoking the same sort of explicit hierarchy.

Siming's special concern (and power) is the balancing of yin and yang. Of particular relevance here is the relation between yin and yang balance and human health, and the importance to individual human health of such balance, as articulated in traditional Chinese medicine. Siming has the power to balance or unbalance yin and yang, and thus to lengthen or shorten human lifespans, or to provide health or prolong illness.

An idea from Daoism/traditional Chinese medicine relates to the Siming. Also known as the sanshi (三尸; "Three Corpses") or sanchong (三蟲; "Three Worms"), the Three Deathbringers are part of a Daoist physiological belief that demonic creatures live inside the human body, and they seek to hasten the death of their hosts. These three supernatural parasites allegedly enter the person at birth, and reside in the three dantian "energy centers" (head, chest, and abdomen). Succeeding the demise of their human host, they become free of the body as malevolent "ghosts". However, this process is regulated by Siming, who may or may not permit this process: in this regard, Siming is said to rely on the reports which the Three Deathbringers (and perhaps Kitchen God) provide on specifically regulated dates. Based on these reports, and perhaps other information, Siming alters the fine yin-yang balance of each individual, thus regulating each one's health or sickness, and ultimately each's lifespan. Nevertheless, humans are believed to have various recourses available to alter the judgement of Siming regarding their fates, such as interfering with the reporting process, praying to Siming or his superiors, or following treatment from a doctor to improve the yin-yang balance, and thus extend the lifespan.