Bonusus was a Bishop of Sardica in the latter part of the fourth century, who taught against the doctrine of the perpetual virginity of Mary. His followers were later labelled "Bonosians" and considered heretical.

Bonusus taught against the prevalent teaching of the Catholic Church in the 4th Century that, after Jesus, Mary had several other children. The Council of Capua in 391, before which the matter was brought, did not pass any judgment on it, but referred it to the Metropolitan Anysius of Thessalonica and the other bishops of Illyria. They condemned Bonosus and tried to exclude him from his church. In a letter to the same bishops Pope Siricius approves the sentence and also condemns the opinion that Mary did not always preserve her virginity.

Notwithstanding his condemnation, and the prudent advice of his study friend St. Ambrose to submit, Bonosus continued to exercise the episcopal functions, to consecrate bishops and ordain priests. According to two letters of Pope Innocent I, one to Martian of Naïssa in 409 and the other to the bishops of Macedonia in 414, those ordained by Bonosus before his condemnation were to be received in the Church without a new ordination, those ordained since Bonosus's condemnation, especially if they had themselves sought to be ordained by him, were to be deprived of their dignity. As Innocent speaks of Bonosus as no longer living, we may infer that he died at the end of the fourth, or the beginning of the fifth century. The baptism conferred by the his followers, the Bonosians, was by some declared valid and by others invalid.

Bonosus can refer to the following figures in ancient history:

A bishop (English derivation of the New Testament Greek επίσκοπος episcopos "overseer, guardian") is an ordained or consecrated member of the Christian clergy who is generally entrusted with a position of authority and oversight. Within the Catholic Church, Eastern Orthodox, Oriental Orthodox, Anglican, Old Catholic, Independent Catholic Churches, and in the Assyrian Church of the East, bishops claim apostolic succession, a direct historical lineage dating back to the original Twelve Apostles. Within these churches, bishops are seen as those who posess the full priesthood and can ordain clergy including other bishops. Some Protestant churches including the Lutheran and Methodist churches have bishops serving similar functions as well, though not always understood to be within apostolic succession in the same way. It is the one ordained deacon, priest and then bishop who is understood to hold the fullness of the (ministerial) priesthood, given responsibility by Christ to govern, teach and sanctify the Body of Christ, members of the Faithful. Priests, deacons, and lay ministers cooperate and assist their bishop(s) in shepherding a flock.