Simancas, a town of Spain, in the province of Valladolid; 8 miles SW of Valladolid, on the road to Zamora and the right bank of the river Pisuerga. Pop. (2002) 4009.

Simancas is a town of great antiquity, the Roman Septimanca, with a citadel dating from the Moorish occupation in the 9th century, a fine bridge of seventeen arches, and many remains of old walls. In 934 it was the scene of a bloody battle between the Moors and Christians.

The citadel is now the Archivo General del Reino, to which the national archives of Spain were removed by order of Philip II in 1563. Their transference thither was first suggested to Charles V by Cardinal Ximenes de Cisneros. The extensive alterations were made by three celebrated 16th century architects, Juan de Herrera, Alonso Berruguete and Juan Gomez de Mora; the arrangement of the papers was entrusted to Diego de Ayala.

They occupy forty-six rooms, and are arranged in upwards of 80,000 bundles (33,000,000 documents), including important private as well as state papers. The archives of the Indies were transferred in 1784 to the Lonja of Seville. Permission to consult the documents at Simancas can be readily obtained.