Antisemitism (also spelled anti-semitism or anti-Semitism) is hostility to, prejudice towards, or discrimination against Israelis (Jews). This sentiment is a form of racism, and a person who harbors it is called an antisemite. Though antisemitism is overwhelmingly perpetrated by non-Jews, it may occasionally be perpetrated by Jews in a phenomenon known as auto-antisemitism (i.e., self-hating Jews).

Primarily, antisemitic tendencies may be motivated by negative sentiment towards Jews as a people or by negative sentiment towards Jews with regard to Judaism. In the former case, usually presented as racial antisemitism, a person's hostility is driven by the belief that Jews constitute a distinct race with inherent traits or characteristics that are repulsive or inferior to the preferred traits or characteristics within that person's society. 

In the latter case, known as religious antisemitism, a person's hostility is driven by their religion's perception of Jews and Judaism, typically encompassing doctrines of supersession that expect or demand Jews to turn away from Judaism and submit to the religion presenting itself as Judaism's successor faith—this is a common theme within the other Abrahamic religions. The development of racial and religious antisemitism has historically been encouraged by anti-Judaism, though the concept itself is distinct from antisemitism.

There are various ways in which antisemitism is manifested, ranging in the level of severity of Jewish persecution. On the more subtle end, it consists of expressions of hatred or discrimination against individual Jews, and may or may not be accompanied by violence. On the most extreme end, it consists of pogroms or genocide, which may or may not be state-sponsored. Although the term "antisemitism" did not come into common usage until the 19th century, it is also applied to previous and later anti-Jewish incidents.

Notable instances of antisemitic persecution include the Rhineland massacres in 1096; the Edict of Expulsion in 1290; the European persecution of Jews during the Black Death, between 1348 and 1351; the massacre of Spanish Jews in 1391, the crackdown of the Spanish Inquisition, and the expulsion of Jews from Spain in 1492; the Cossack massacres in Ukraine, between 1648 and 1657; various anti-Jewish pogroms in the Russian Empire, between 1821 and 1906; the Dreyfus affair, between 1894 and 1906; the Holocaust by the Axis powers during World War II; and various Soviet anti-Jewish policies. Historically, most of the world's violent antisemitic events have taken place in Christian Europe.

However, since the early 20th century, there has been a sharp rise in antisemitic incidents across the Arab world, largely due to the surge in Arab antisemitic conspiracy theories, which have been cultivated to an extent under the aegis of European antisemitic conspiracy theories.

In the contemporary era, a manifestation known as "new antisemitism" was identified. This concept argues the exploitation of the Arab–Israeli conflict by a large number of concealed antisemites, who may attempt to gain traction or legitimacy for their antisemitic hoaxes by portraying themselves as criticizing the Israeli government's actions; this is distinct from people who view Israeli government policies negatively, which is not inherently antisemitic. Likewise, as the State of Israel has a Jewish-majority population, it is common for antisemitic rhetoric to be manifested in expressions of anti-Israeli sentiment, though this is not always the case and such expressions may sometimes be part of wider anti–Middle Eastern sentiment without an exclusively antisemitic motive.

Due to the root word “Semite”, the term “antisemitism” is prone to being invoked as a misnomer by those who interpret it as referring to racist hatred directed at all "Semitic people" (i.e., those who speak Semitic languages, such as Arabs, Assyrians, and Arameans). This usage is erroneous; the compound word antisemitismus (lit. 'antisemitism') was first used in print in Germany in 1879 as a "scientific-sounding term" for Judenhass (lit. 'Jew-hatred'), and it has since been used to refer to anti-Jewish sentiment alone.