Reliable evidence indicates that the following popes were martyred.
1. Pope St. Peter the Apostle of Rome (33-64)
Pope St. Peter, the Prince of the Apostles, was crucified upside-down in Rome (Kirsch).
2. Pope St. Anacletus of Rome (76-4/26/88)
“We know he died a martyr” (Campbell).
3. Pope St. Clement I of Rome (88-11/23/97)
Rufinus in 400 is the first to mention the martyrdom of Pope St. Clement I (Chapman). In 417 Pope St. Zosimus [Letter 2] said St. Clement gave his life to testify to the faith he learned from St. Peter (ibid.). Predestinatus called him a martyr in 430, as did the 442 Synod of Vaison (ibid.). That he died in exile is supported by the absence of a tradition that he was interred in Rome (ibid.). St. Constantine-Cyril, Apostle to the Slavs (827-2/14/869), discovered his holy relics and Pope Adrian II (12/14/867-12/14/872) placed them “in the high altar of the basilica of St. Clement Rome” (ibid.)
4. Pope St. Sixtus I of Rome (115-4/6/125)
“The ‘Felician Catalogue’ of popes and the various martyrologies give him the title of martyr” (Ott).
5. Pope St. Telesphorus of Rome (9/30/125-1/2/136)
Bishop St. Irenaeus Irenaeus of Lyons informs us [Against Heresies 3:3:3] that Pope St. Telesphorus of Rome “was gloriously martyred.”
6. Pope St. Callistus I of Rome (217-10/14/222)
That Pope St. Callistus I “is the earliest pope found in the fourth-century ‘Depositio Martirum‘ … is good evidence that he was really a martyr” (Chapman).
7. Pope St. Urban I of Rome? (10/14/222-5/25/230)
Pace Giovanni Battista de Rossi, Pope St. Urban I of Rome is one of the martyrs who was buried in the Catacomb of Praetextatus (Kirsch).
8. Pope St. Pontian of Rome (7/21/230-9/28/235)
Together with St. Hippolytus, “he died in consequence of the privations and inhuman treatment he had to bear” on the island of Sardinia, to which he was exiled by the tyrannical Roman Emperor Maximinus the Thracian (Kirsch).
9. Pope St. Fabian of Rome (1/10/236-1/20/250)
At the start of the persecution under the Roman Emperor Decius, Pope St. Fabian was martyred (Meier). His Greek epitaph, discovered in 1850 by Giovanni Battista de Rossi, reads “Fabian, bishop and martyr” (ibid.).
10. Pope St. Cornelius of Rome (3/251-6/253)
Bishop St. Cyprian the Martyr of Carthage (†259) “repeatedly calls” Pope St. Cornelius of Rome a martyr, and the Latin inscription on the catacomb of the holy pope says “CORNELIUS* MARTYR*” (Chapman).
11. Pope St. Sixtus II of Rome (8/30/257-8/6/258)
He was beheaded on a chair in Rome (Ott).
12. Pope St. Marcellus I of Rome (306-1/16/309)
Pope St. Damasus I of Rome (10/1/366-12/11/384) says that Pope St. Marcellus I died shortly after being exiled from Rome by the tyrant Maxentius (Kirsch).
13. Pope St. Eusebius of Rome (309-310)
He died in exile for strictly enforcing Church discipline, and Pope St. Damasus I of Rome calls him a martyr in his epitaph (Kirsch).
14. Pope St. John I of Rome (523-526)
“The Latin Church has placed him among its martyrs” (Clugnet)
15. Pope St. Silverius of Rome (536-537)
After being “unlawfully deposed,” Pope St. Silverius of Rome “died in consequence of the privations and harsh treatment he endured” (Kirsch).
16. Pope St. Martin I of Rome (7/5/649-9/16/655)
This wonderworking pope died in exile for refusing to ratify the Type of the tyrannical Byzantine Emperor Constans II (Mershman).