"Who is a Jew?" (Hebrew: מיהו יהודי pronounced [ˈmi(h)u je(h)uˈdi]) is a basic question about Jewish identity and considerations of Jewish self-identification. The question pertains to ideas about Jewish personhood, which have cultural, ethnic, religious, political, genealogical, and personal dimensions. Orthodox Judaism and Conservative Judaism follow Jewish law (Halakha), deeming people to be Jewish if their mothers are Jewish or if they underwent a halakhic conversion. Reform Judaism and Reconstructionist Judaism accept both matrilineal and patrilineal descent as well as conversion. Karaite Judaism predominantly follows patrilineal descent as well as conversion.

Jewish identity is also commonly defined through ethnicity. Opinion polls have suggested that the majority of Jews see being Jewish as predominantly a matter of ancestry and culture, rather than religion.