Diego Deza (1444 — 1523) was a theologian and inquisitor of Spain. He was one of the more notable figures in the Spanish Inquisition, and succeeded the notorious Tomás de Torquemada to the post of Grand Inquisitor.

Deza was born in Toro and entered the Dominican Order at a young age. He held a number of ecclesiastical posts, and also tutored one of the sons of King Ferdinand and Queen Isabella. After first serving as Bishop of Zamora, of Salamanca, and of Jaén, he became Archbishop of Seville in 1505. Deza was commissioned as Grand Inquisitor for Castile, Leon, and Granada on November 24, 1498. On September 1 the following year, his remit was expanded to cover the whole of Spain.

Deza was the successor to Tomás de Torquemada, perhaps the most famous of all inquisitors. Like Torquemada, Deza had a particular dislike of conversos — Jews or Muslims who had converted to Christianity but who were often accused of secretly retaining their original faith. Also like Torquemada, Deza was accused of being overzealous in his work, and of showing excessive cruelty - his reputation was sufficient that in 1507, the Pope was forced to publicly request moderation. Accusations were also made that Deza used his position to enrich himself, confiscating the wealth of accused heretics for himself. A complaint about Deza, made to the royal secretary by Captain Gonzola de Avora, said that Deza and his lieutenants "have no regard either for God or for justice; they kill, steal, and dishonor girls and women to the disgrace of the Christian religion."

Deza himself was later accused of secretly practicing Judaism, a charge mainly based on the fact that he himself had Jewish blood on his mother's side. The accusation was probably political, but nevertheless damaged his standing somewhat. His position was further undermined by several open insurrections against the Inquisition, particularly against his chief lieutenant Diego Rodriguez Lucero . Lucero, the Inquisitor of Cordoba, was even more hated than Deza, and in late 1506, had narrowly escaped with his life when an angry mob stormed the Inquisition's base in Cordoba and freed all its prisoners. The King decided that Deza had become a liability, and Deza was forced to resign in 1507.

Considerably later in his life, Deza was named Bishop of Toledo (and thus Primate of Spain), but was not able to take up the position due to illness. He died on July 9, 1523.