was a philosopher and astronomer, who, in the beginning of the 9th century, ascended the Almamon throne of the caliphs of Bagdad. He was the son of Harun Al-Rashid, and grandson of Almanfor. His name is otherwise written Mamoon, Almamon, Almamoon, or Al-Maimoon. Having been educated with great care, and with a love for the liberal sciences, he applied himself to cultivate and encourage them in his own country. For this purpose he requested the Greek emperors to supply him with such books on philosophy as they had among them; and he collected skilful interpreters to translate them into the Arabic language. He also encouraged his subjects to study them; frequenting the meetings of the learned, and assailing at their exercises and deliberations. He caused Ptolemy's Almagest to be translated in 827, by Isaac Ben-homai, and Thabet Ben-korah, according to Herbelot, but, according to others, by Sergius, and Alhazen the son of Joseph. In his reign, and doubtless by his encouragement, an astronomer of Bagdad, named Habaffi, composed three sets of astronomical tables.

Almamon himself made many astronomical observations, and determined the obliquity of the ecliptic to be then 23° 35' (or 23° 33' in some manuscripts), but Voisin says 23° 51' or 23° 34'. He also caused skilful observers to procure proper instruments to be made, and to exercise themselves in astronomical observations; which they did accordingly at Shemasi in the province of Bagdad, and upon Mount Cænus near Damus.

Under the auspices of Almamon also a degree of the meridian was measured on the plains of Sinjar or Sindjar (or, according to some, Pinjar), upon the borders of the Red Sea; by which the degree was found to contain 66½ miles, of 4000 coudees each, the coudee being a foot and a half; but it is not known what foot is here meant, whether the Roman, the Alexandrian, or some other. Riccioli makes this measure of the degree amount to 81 ancient Roman miles, which value answers to 62,046 French toises; a quantity more than the true value of the degree by almost one-third. Finally, Almamon revived the sciences in the East to such a degree, that many learned men were found, not only in his own time, but after him, in a country where the study of the sciences had been long forgotten. This learned king died near Tarsus in Cilicia, by having eaten too freely of some dates, on his return from a military expedition, in the year 833.