Religious syncretism is the blending of religious belief systems into a new system, or the incorporation of other beliefs into an existing religious tradition.

This can occur for many reasons, where religious traditions exist in proximity to each other, or when a culture is conquered and the conquerors bring their religious beliefs with them, but do not succeed in eradicating older beliefs and practices.

Many religions have syncretic elements, but adherents often frown upon the application of the label, especially those who belong to "revealed" religions, such as Abrahamic religions, or any system with an exclusivist approach, seeing syncretism as corrupting the original religion. Non-exclusivist systems of belief on the other hand feel more free to incorporate other traditions into their own.

Classical Athens was exclusive in matters of religion. Some sources assert that the Decree of Diopeithes made the introduction of and belief in foreign gods a criminal offense and allowed only Greeks to worship in Athenian temples and festivals (as foreigners were considered impure). Other sources dispute the existence of the decree.

Syncretism functioned as a feature of Hellenistic Ancient Greek religion, although only outside of Greece. Overall, Hellenistic culture in the age that followed Alexander the Great itself showed syncretist features, essentially blending Mesopotamian, Persian, Anatolian, Egyptian, and (eventually) Etruscan–Roman elements within a Hellenic formula. The Egyptian god Amun was developed into the Hellenized Zeus Ammon after Alexander the Great went into the desert to seek out his oracle at the Siwa Oasis.

The Romans, identifying themselves as joint heirs to a very similar civilization, identified Greek gods with similar figures in the Etruscan-Roman tradition, though usually without copying cult practices. (For details, see interpretatio graeca.) Syncretic gods of the Hellenistic period also found wide favor in Rome. For example, Serapis, Isis, and Mithras. Cybele, as worshipped in Rome, essentially represented a syncretic East Mediterranean goddess. The Romans imported the Greek god Dionysus into Rome, where he merged with the Latin mead god Liber, and converted the Anatolian Sabazios into the Roman Sabazius.

The degree of likeness varied: Jupiter was perhaps a better match with Zeus than the rural huntress Diana was for the feared Artemis. Ares was not homologous to Mars. The Romans physically imported the (statue of) Anatolian goddess Cybele into Rome from her cult center in Anatolian Pessinos in the form of her original aniconic archaic stone idol; they identified her as Magna Mater and gave her a matronly, explicitly iconic image developed in Hellenistic Pergamum.

Likewise, when the Romans encountered Celts and Germanic peoples, they mingled those peoples' gods with their own, creating Sulis Minerva, Apollo Sucellos (Apollo the Good Smiter) and Mars Thingsus (Mars of the war-assembly), among many others. In the Germania, the Roman historian Tacitus speaks of Germanic worshippers of Hercules and Mercury; most modern scholars tentatively identify Hercules as Thor and Mercury as Odin.