Joniškis (pronunciationⓘ; Samogitian: Juonėškis; Polish: Janiszki) is a city in northern Lithuania with a population of about 9,900.

Historical versions of the name in other languages include Polish: Janiszki, Russian: Янишки Yanishki, Yiddish: יאַנישאָק Yanishok, Latvian: Jānišķe, and German: Jonischken.

It was mentioned in written sources on 23 February 1536 when Bishops of Vilnius and Samogitia visited the area and found that people still practiced the old pagan faith.

The bishop of Vilnius, John of the Lithuanian Dukes baptized the locals and established a new parish on 23 February 1536.

493 people were murdered in total by an Einsatzgruppen of Joniškis policemen and Lithuanian nationalists supervised by the Germans.