Marcus I ( el, Μᾶρκος Αʹ; died 211) was the

bishop of Byzantium The ecumenical patriarch ( el, Οἰκουμενικός Πατριάρχης, translit=Oikoumenikós Patriárchēs) is the archbishop of Constantinople (Istanbul), New Rome and ''primus inter pares'' (first among equals) among the heads of the ...

for 13 years (198–211 AD). He succeeded Bishop Olympianus. His term of office took place during the persecution of Christians by Emperor Septimius Severus. His successor was Philadelphus.

Sources

Marcus I page on Ecumenical Patriarchate website
2nd-century Romans 3rd-century Romans 2nd-century Byzantine bishops 3rd-century Byzantine bishops Roman-era Byzantines Bishops of Byzantium 211 deaths Year of birth unknown {{EarlyChurch-bishop-stub