Abu'l-Qasim Ja'far ibn Muhammad ibn Fasanjus

Abu'l-Qasim Ja'far ibn Muhammad ibn Fasanjus, was an Iranian statesman from the Fasanjus family who served the Buyid dynasty.

Ja'far was born in Baghdad in 965/6. He was the son of Abu'l-Faraj Muhammad, who had served as a de facto vizier (without the actual title) in 963–967, and as full vizier in 970–971.

Ja'far was appointed by the Buyid ruler Sultan al-Dawla as his vizier in 1018/9, after the flight of the previous incumbent, al-Hasan ibn Fadl ibn Sahlan. Ja'far held the office only briefly, as he and his brothers were arrested in autumn 1019.

Ja'far died at Khuzistan in 1029. He had a son known as Dhu'l-Sa'adat, who served as vizier under the Buyid ruler Abu Kalijar until his execution in February 1049.