A mikveh or Mikvah (Hebrew: מִקְוֶה / מקווה, Modern: mīqve, Tiberian: mīqwe, pl. mikva'ot, mikvot, or (Ashkenazic) mikves, lit., "a collection") is a bath used for ritual immersion in Judaism to achieve ritual purity.

In Orthodox Judaism, these regulations are steadfastly adhered to; consequently, the mikveh is central to an Orthodox Jewish community. Conservative Judaism also formally holds to the regulations. The existence of a mikveh is considered so important that, according to halacha, a Jewish community is required to construct a kosher mikveh even before building a synagogue, and must go to the extreme of selling Torah scrolls, or even a synagogue if necessary, to provide funding for its construction.

Outside of Judaism, mikveh has its counterpart in Christianity, called baptism, though Christian baptism is one of the main requirements for conversion to Christianity, while the Jewish mikveh focuses mainly on ritual purity.

Formed from the Semitic root ק-ו-ה (q-w-h, "collect"). In the Hebrew Bible, the word is employed in the sense of "collection", including in the phrase מקוה המים (miqwêh hammayim, "collection of water") in Genesis 1:10; Exodus 7:19; and Leviticus 11:36. Ben Sira is the earliest author to use מקוה as a word for "pool" (Ecclus 43:20, 48:17), and the Mishnah is the earliest text to use it in the sense of "ritual bath".

Before the beginning of the 1st century BCE, neither written sources nor archaeology gives any indication about the existence of specific installations used for ritual cleansing. Mikvot appear at the beginning of the 1st century BC, and from then on, ancient mikvot can be found throughout the Land of Israel, as well as in historic communities of the Jewish diaspora. Hundreds of mikvot from the Second Temple period have been discovered so far across the Land of Israel, in locations including Jerusalem, Hebron, Masada, and Hannaton.

The absence of dedicated mikvot prior to the 1st century BCE is surprising, given that laws of purification were in fact observed by many Jews in earlier periods, as indicated by Biblical narratives and the Elephantine papyri. One suggestion is that Jews used natural water sources such as springs for immersion, rather than building dedicated mikvot. Alternatively, according to many Halakhic authorities the prohibition on using pumped water for a mikveh is rabbinic, not biblical. Prior to the creation of such a rabbinic decree around 100 BCE,^{[dubious – discuss]} Jews may have immersed in above-ground basins that were built as part of buildings, or affixed to the roofs of buildings, and filled manually. Such structures, dating to the First Temple period, have been discovered in ancient Ashdod and possibly in Dan. The reason for such a rabbinic decree may have been to distance the practice of ritual immersion from the culture of bathhouses, which spread through the region during the Hellenistic period.

The traditional rules regarding the construction of a mikveh are based on those specified in classical rabbinic literature. Numerous biblical laws indicate that one must "bathe their flesh in water" to become purified from ritual impurity. The type of bathing is specified in Leviticus 11:36, which states that "a spring, or a cistern, a gathering (mikveh) of water" is a source of purity. A mikveh must be built into the ground or built as an essential part of a building. Portable receptacles, bathtubs, whirlpools, or jacuzzis cannot therefore function as mikvot.

However, many Sephardic communities, as well as Ashkenazi Jews in America before World War 2, customarily allowed mikvehs to be filled using municipal water. Bans on such practices only became common in the US after an influx of European Ashkenazi rabbis, who saw the use of municipal water as too lenient. Some rabbis considered permitting spas to be used, but ultimately decided against it as it may encourage women to prefer warm water during immersion instead of prioritizing cold water. According to Rabbi Isaac Esrig, in 1957 most American mikvaot were filled using municipal water.