The Sabians, sometimes also spelled Sabaeans or Sabeans, are a mysterious 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 pagan 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).

Another important religious group who adopted the epithet were the Mandaeans (the Mandaean Sabians), a Gnostic sect living in the marshlands of southern Iraq.

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.^: 1 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.^: 45