Konstanty Wasyl Ostrogski (2 February 1526 – 13 (23) February 1608, Belarusian: Канстантын Васiль Астрожскi Lithuanian: Konstantinas Vasilijus Ostrogiškis Ukrainian: Костянтин-Василь Острозький) was a magnate of the Polish-Lithuanian Commonwealth, a prince, starost of Volodymyr-Volynskyi, marshal of Volhynia and voivode of the Kiev Voivodeship. Ostrogski refused to help False Dmitriy I and supported Jan Zamoyski.

In 1570s he waged a war against another magnate, Stanisław Tarnowski, about disputed possession of estates in the area of Tarnów, in Lesser Poland.

Prince Ostrogski was of Eastern Orthodox faith and he was active in supporting the illegal Orthodox Church (see Union of Brest) and, yet before that, promoting the Eastern culture in the Polish–Lithuanian Commonwealth. In 1576 he established the Ostroh Academy, a regarded humanist educational and scholarship institution, with the instruction in Greek, Latin and Old Church Slavonic languages. In 1581 the Academy produced and published the Ostrog Bible, the first complete printed edition of the Bible in Old Church Slavonic.

Ostrogski's huge latifundium, or landed estate in the eastern Polish–Lithuanian Commonwealth, consisted of 100 towns and 1300 villages. It was Ostrogski who built Starokostiantyniv Castle.

While Konstanty Wasyl Ostrogski was the proponent of the Eastern Orthodox religion, his son Janusz-Ivan converted to Roman Catholicism.

He got married on January 1553 in Tarnów.