Diego de Simancas or Diego de Simancas Simancas (died 1583) was a Roman Catholic prelate who served as Bishop of Zamora (1578–1583), Bishop of Badajoz (1568–1578), and Bishop of Ciudad Rodrigo (1564–1568).

[4] On 15 December 1564, he was appointed during the papacy of Pope Pius IV as Bishop of Ciudad Rodrigo.

[2][3] On 3 December 1568, he was appointed during the papacy of Pope Pius V as Bishop of Badajoz.

[1][2][3] On 13 June 1578, he was appointed during the papacy of Pope Gregory XIII as Bishop of Zamora.

[1][2][3] He served as Bishop of Zamora until his death on 16 October 1583.