Mar Shimun Bar Sabbae (Classical Syriac: ܡܪܝ ܫܡܥܘܢ ܒܪܨܒܥܐ, romanized: Shemʿon bar Ṣabbaʿe;[1] died Good Friday, 345) was the Chaldean Bishop of Seleucia-Ctesiphon, from Persia, the de facto head of the Church of the East, until his death.

He was bishop during the persecutions of King Shapur II of the Sasanian Empire of Iran, and was executed along with many of his followers.

[3] A variation of the name is also attested in the Dead Sea scrolls and is a sobriquet given Joseph Barsabbas.

In 316, he had been named coadjutor bishop of his predecessor, Papa bar Gaggai, in Seleucia-Ctesiphon (now al-Mada'in).

It is also thought that a reason was that Shimoun had converted the kings mother Ifra Hormizid (who was of Jewish descent) to Christianity.