Abu Sa'id Ahmed ibn Mohammed ibn Abd Jalil Sijzi (short for Sijistani) (Persian: ابوسعید سجزی‎) was a Persian astronomer and mathematician from Sistan, a region lying in the south-west of Afghanistan and south-east of Iran.

Sijzi is thought to have been born around 945 CE, and lived through about 1020. His main scientific focus was astronomy. He had a deep knowledge of literature which he used to his advantage. He dedicated work to 'Adud al-Daula and the prince of Balkh. He also worked in Shiraz making astronomical observations from 969 to 970. He also did a lot of geometry work.

Biruni wrote that Sijzi believed in a heliocentric system in which the Earth was moving and that he invented an astrolabe called the "Zuraqi" based on this idea:

Sijzi was a mathematician who made a special study of the intersections of conic sections and circles. He replaced the old kinematical trisection of an angle by a purely geometric solution (intersection of a circle and an equilateral hyperbola.)