[a][2] He is sometimes identified with the logician and herbalist Peter of Spain (Latin: Petrus Hispanus; Portuguese: Pedro Hispano), which would make him the only pope to have been a physician.

In March 1273, he was elected Archbishop of Braga, but did not assume that post; instead, on 3 June 1273, Pope Gregory X created him Cardinal Bishop of Tusculum (Frascati).

On 14 May 1277, while the pope was alone in this apartment, the ceiling collapsed; John was rescued alive from beneath the rubble; however, he died of his serious injuries on 20 May, possibly an early recorded case of crush syndrome.

In the 19th century, the Duke of Saldanha, as Portuguese Ambassador to the Holy See, had the pope's remains transferred to a new sarcophagus sculpted by Filippo Gnaccarini.

[9][10] After his death, it was rumored that John XXI had actually been a necromancer, a suspicion frequently directed towards the few scholars among medieval popes (see, e.g., Sylvester II).

[11] Since the works of "Peter of Spain" continued to be studied and appreciated, however, Dante Alighieri placed "Pietro Spano" in his Paradiso's Sphere of the Sun with the spirits of other great religious scholars.