Hegesippus (Greek Hēgēsippos) was a Jewish-Christian convert of the mid-2nd century, originally from Palestine, who travelled extensively across the Christian world of his day — from Palestine through Syria, through Corinth (where he stayed with the Christian community), and ultimately to Rome where he completed his work during the pontificate of Anicetus and remained until the pontificate of Eleuterus (c. 155–180).
His five-book Hypomnemata ('Memoirs') is the lost masterpiece of foundational Christian historiography. The work — which Eusebius preserves in substantial fragments — is the earliest extant systematic Christian account of the apostolic succession of bishops in major Christian sees (Jerusalem, Antioch, Rome, Corinth), of the early Jewish-Christian community of Jerusalem, of the martyrdoms of James the brother of the Lord and of Symeon of Jerusalem (see Symeon's entry above), and of the early Christian engagement with rival Jewish, gnostic, and pagan religious movements.
Hegesippus's foundational contribution was the methodological insight that the historicity and authenticity of Christian doctrine could be established by tracing the continuous succession of bishops in specific apostolic-foundation sees back to the original apostolic teaching. The principle was developed a generation later by Irenaeus of Lyon into the systematic Catholic doctrine of apostolic succession that remains the foundational Catholic understanding to this day.