False Dmitriy I
Konstanty Wasyl Ostrogski (2 February 1526 – 13 or 23 February 1608, also known as "Kostiantyn Vasyl Ostrozky", , , ) was a Ruthenian Orthodox 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.