Prince Janusz Ostrogski (Lithuanian: Jonušas Ostrogiškis) (1554 – 17 September 1620 in Tarnów) was a Polish-Lithuanian noble and statesman.

He served as a voivode of Volhyn (1584-1593), as a castellan of Kraków (from 1593 on), and as a starosta of Bohuslav (from 1591), Biała Cerkiew (since 1592), Czerkasy and Kaniów (from 1594), Perejasław (1604 on) and Włodzimierz.

For the protection of state borders and their own possessions in 1609, he founded Ostrogski ordination, the capital of which over time became Dubno.

He held several senior government positions; opposed the support of the Pretender Dmitri-I, and the Commonwealth war with the Moscow State (1609-1618),[5] strengthened Dubno Castle ramparts, built a deep moat and a suspension bridge in the city founded by the Bernardine church and church of St. John of Nepomuk.

Cherished treasures of the ancestors, especially the prized gold medal with the image of his father, Prince Constantine-Basil, which as amulets took with him on hikes.