The Sabians, sometimes also spelled Sabaeans or Sabeans, are a religious group mentioned three times in the Quran (as الصابئون al-Ṣābiʾūn, in later sources الصابئة al-Ṣābiʾa), where it is implied that they belonged to the 'People of the Book' (ahl al-kitāb). Their original identity, which seems to have been forgotten at an early date, has been called an "unsolved Quranic problem". Modern scholars have variously identified them as Mandaeans, Manichaeans, Sabaeans, Elchasaites, Archontics, ḥunafāʾ (either as a type of Gnostics or as "sectarians"), or as adherents of the astral religion of Harran. Some scholars believe that it is impossible to establish their original identity with any degree of certainty.

At least from the ninth century on, the Quranic epithet 'Sabian' was claimed by various religious groups who sought recognition by the Muslim authorities as a People of the Book deserving of legal protection (dhimma). Among those are the Sabians of Harran, adherents of a poorly understood ancient Semitic religion centered in the upper Mesopotamian city of Harran, who were described by Syriac Christian heresiographers as star worshippers. These Harranian Sabians practiced an old Semitic form of polytheism, combined with a significant amount of Hellenistic elements. Most of the historical figures known in the ninth–eleventh centuries as al-Ṣābiʼ were probably either members of this Harranian religion or descendants of such members, most notably the Harranian astronomers and mathematicians Thabit ibn Qurra (died 901) and al-Battani (died 929).

From the early tenth century on, the term 'Sabian' was applied to purported 'pagans' of all kinds, such as to the ancient Egyptians and Greeks, or to Buddhists. Ibn Wahshiyya (died c. 930) used the term for a type of Mesopotamian paganism that preserved elements of ancient Assyro-Babylonian religion.

Today in Iraq and Iran, the name 'Sabian' is normally applied to the Mandaeans, a modern ethno-religious group who follow the teachings of their prophet John the Baptist (Yahya ibn Zakariya). These Mandaean Sabians, whose most important religious ceremony is baptism, are monotheistic, and their holy book is known as the Ginza Rabba. Mandaean Sabian prophets include Adam, Seth, Noah, Shem and John the Baptist with Adam being the founder of the religion and John being the greatest and final prophet.

The etymology of the Arabic word Ṣābiʾ is disputed. According to one interpretation, it is the active participle of the Arabic root ṣ-b-ʾ ('to turn to'), meaning 'converts'. Another widely cited hypothesis, first proposed by Daniel Chwolsohn in 1856, is that it is derived from an Aramaic root meaning 'to dip' or 'to baptize'.

The interpretation as 'converts' was cited by various medieval Arabic lexicographers and philologists, and is supported by a tradition preserved by Ibn Hisham (died 834, editor of the earliest surviving biography of Muhammad) relating that the term ṣābiʾa was applied to Muhammad and the early Muslims by some of their enemies (perhaps by the Jews), who regarded them as having 'turned' away from the proper religion and towards heresy. As such, the term may have been reappropriated by early Muslims, first as a self-designation and then to refer to other people from a Jewish Christian background who 'turned' to the new revelations offered by Muhammad. In the context of the Quranic passages in which the term occurs, it may thus refer to all people who leave their faiths, finding fault in them, but who have yet to come to Islam. In this sense, the term ṣābiʾ would be similar in meaning to the term ḥanīf.

Understanding the term as a reference to 'dippers' or 'baptizers' fits best with those interpretations that identify the Quranic Sabians with baptist sects like the Elchasaites or the Mandaeans. However, this etymology has also been used to explain Ibn Hisham's story about Muhammad and his followers being called 'Sabians', which would then be a reference to the ritual washing performed by Muslims before prayer, a practice resembling those of various baptist sects.

Other etymologies have also been proposed. According to Judah Segal, the term referred to Ṣōbā, a Syriac name for Nisibis, a city in Upper Mesopotamia. It has also been related to Hebrew ṣābā', "[heavenly] host", implying star worshippers.