Spiritual Assembly is a term given by ʻAbdu'l-Bahá to refer to elected councils that govern the Baháʼí Faith. Because the Baháʼí Faith has no clergy, they carry out the affairs of the community. In addition to existing at the local level, there are national Spiritual Assemblies (although "national" in some cases refers to a portion of a country or to a group of countries). Spiritual Assemblies form part of the elected branch of the Baháʼí administration.

Baháʼu'lláh, ʻAbdu'l-Bahá, and Shoghi Effendi stated how Spiritual Assemblies should be elected by the Baháʼís, defined their nature and purposes, and described in considerable detail how they should function. Since these institutions are grounded in the Baháʼí authoritative texts, Baháʼís regard them as divine in nature, and contrast the wealth of scriptural guidance with the paucity of scriptural texts on which Jewish, Christian, and Islamic religious institutions are based.

The Universal House of Justice has added that among the responsibilities of Local Spiritual Assemblies are to be “channels of divine guidance, planners of the teaching work, developers of human resources, builders of communities, and loving shepherds of the multitudes.” On a practical level, they organize local Baháʼí communities by maintaining a local Baháʼí Fund, owning the local Baháʼí center (if one exists), organizing Baháʼí events, counseling Baháʼís about personal difficulties, assisting with Baháʼí marriages and funerals, providing educational programs to adults and children, publicizing the Baháʼí Faith locally, fostering projects for the social and economic development of the region, and enrolling new members of the religion. Spiritual Assemblies appoint individuals, task forces, and committees to carry out many of their functions. National Spiritual Assemblies have a similar mandate at the national level: they coordinate publishing and distribution of Baháʼí literature, direct relations with national organizations and governmental agencies, oversee the work of local spiritual assemblies, and (in some countries) Regional Councils, set local Baháʼí jurisdictional boundaries, provide various educational services and programs, and set the overall tone and direction of the national community.

The origin of the institution of the local Spiritual Assembly originates from Baháʼu'lláh's book of laws, the Kitáb-i-Aqdas:

The passage gives the institution a name, a minimum number (nine, for “the number of Baha” refers to the numerical value of the letters of that word, which is nine), and a general responsibility to take care of the welfare of others even as they would take care of their own. While the resulting institution is local, in the Kitáb-i-Aqdas Baháʼu'lláh also spoke about the responsibilities of the supreme or Universal House of Justice. In response to the passage, Mírzá Asadu'lláh Isfahání, a prominent Baháʼí teacher, organized an unofficial Baháʼí consultative body in Tehran, Iran, about 1878. The first official Baháʼí consultative body was organized under ʻAbdu'l-Bahá's direction by Hand of the Cause Hají Ákhúnd in Tehran in 1897; by 1899 it was an elected body. Because of the difficulties in Iran caused by persecution of the Baháʼí Faith, the Tehran body served to coordinate both local and national Baháʼí activities. It is not known what name the body was organized under.

The development of a Baháʼí community in the United States in the 1890s necessitated the creation of local Baháʼí consultative bodies there. In 1899 the Baháʼís of Chicago elected a local council based on their awareness of the provisions of the Kitáb-i-Aqdas (which was circulated in provisional English translation as a typescript as early as 1900). The New York Baháʼís elected a “Board of Counsel” in December 1900. In 1901 the Chicago body was reorganized and re-elected and took the name “House of Justice of Bahais of Chicago, Ills.” In response, ʻAbdu'l-Bahá revealed three tablets of encouragement and guidance to the body, including prayers to say at the beginning and end of their meetings, prayers that Baháʼís use around the world today for their Spiritual Assembly meetings.

In 1902 ʻAbdu'l-Bahá sent a very important tablet to the Chicago governing body where he said "let the designation of that body be 'Spiritual Assembly'—this for the reason that, were it to use the term 'House of Justice', the government might hereafter come to suppose that it was acting as a court of law, or concerning itself in political matters, or that, at some indeterminate future time, it would involve itself in the affairs of government.... This same designation hath been universally adopted throughout Iran." For this reason, Baháʼí local and national governing bodies are designated “Spiritual Assemblies” to this day.

The first decade of the twentieth century saw the proliferation of local Baháʼí governing bodies. Often unaware of ʻAbdu'l-Bahá's guidance, they had a variety of titles in English and Persian, such as “Council Board, “Board of Consultation,” “House of Spirituality,” and "Executive Committee." Unaware ʻAbdu'l-Bahá had told the Chicago Baháʼís to elect their body every five years, they were usually elected annually or even semi-annually. The number of members varied from five to nineteen (except in New York City, where ʻAbdu'l-Bahá, in 1911, said they should elect twenty-seven members in order to be inclusive of and to foster unity between that city's diverse Baháʼí groups). They were male only until ʻAbdu'l-Bahá said, in 1911, that women should be elected to the local governing bodies existing in the United States; their exclusion from local bodies continued in Iran until the 1950s, because of Iranian cultural conventions. In the period of 1900 - 1911, consultative bodies are known to have existed in Kenosha, Wisconsin, Boston, Massachusetts, Washington, D.C., Spokane, Washington, northern Hudson County, New Jersey, the greater San Francisco area, California, in the United States; and in Bombay, British Raj India; Cairo, Khedivate of Egypt; Acre, Ottoman Syria; Baku, Tbilisi, Ashgabat and Samarqand in the Russian Empire; and Mashhad, Abadih, Qazvin, and Tabriz, Persia. Consultative bodies also existed for the Jewish and Zoroastrian Baháʼís in Tehran and for the women of a few Baháʼí communities.