The Judaizing teachers were a group of Jewish converts to Christianity who felt that converts to Christianity must convert to Judaism first.

This group was very active in the church of the 1st century CE. They taught that before Gentiles could accept Christianity that they first needed to accept the basic principles of Judaism, including the dietary laws and the practice of male circumcision. These requirements made Christianity a much less appealing religious choice to many Gentiles.

Paul saw these teachers as being both dangerous to the spread of Christianity and propagators of grievous doctrinal error. Many of his letters included in the New Testament (the so-called Pauline epistles) contain considerable material disputing the view of this group and condemning its practitioners. Paul stated that the death of Christ had essentially brought a conclusion to the Law of Moses by its finally having been completely fulfilled once and for all. He condemned Peter for his seemingly ambivalent reaction to the Judiaizers, embracing them publicly in places where their concepts were popular while holding the private opinion that teachings were erroneous.

Judaizing teachers are even more strongly condemned in the Epistle of Barnabas. (Although it did not become part of the New Testament canon, it was widely circulated among Christians in the first two centuries.) Whereas Paul acknowledged that the Law of Moses and its observance had served a good purpose up until the time of Christ, the Epistle of Barnabas condemns most Jewish practices, claiming that Jews had grossly misunderstood and misapplied the Law of Moses.

The term "Judaize" is also employed as a condemnatory one in some English translations of the Qur'an as voicing Muhammad's displeasure with those who converted to Judaism instead of converting to Islam.

A minority opinion held by Messianic Judaism and other Christians is that the Judaizing teachers claimed that circumcision and full obedience to the Torah were required for salvation. As the Apostles argued it was not necessary for salvation, Messianic and Nazarene Judaism generally believe that it would be eventually expected, if the converts intended to become Jews, as the gentiles joined first and then learned what would be expected of them at some point after studying the Law of Moses.