Žižka's oak, Ancient oak tree in Náměšť nad Oslavou, Czech Republic.

Žižka's oak is a summer oak in Náměšť nad Oslavou that reaches about 20 meters in height with a massive trunk around 10 meters in circumference. The interior of the trunk is hollow and has been sealed with a shingled roof.

The oak is a remnant of the original oak woods that once covered the area, not a planted tree. Local priest R. Tenora estimated its age at 800–1000 years in 1910; it is now often given as about 900 years old. In 2019 it reached the final of the Tree of the Year contest and placed second after the Chudobín pine, whose nominating association ceded the prize (tree treatment and inspection) to the oak.

Legends link the oak to Jan Žižka's Moravian campaign: he is said to have slept beneath it or tethered his horse to it. No year is given, and historical records show the castle was taken in 1408 by Lacko of Kravaře, not by Žižka.

The oak stands in a sharp bend of the road near the château, only about 8 metres from the asphalt. It is not open to the public, but it can be seen well from the road, so there is no need to enter the grounds.

The hollow trunk has openings in several places, and the crown is now formed by only two main branches. In 2019 the tree placed second in the Tree of the Year contest, after the Chudobín pine; that tree's sponsor agreed to pass the prize, treatment and a health check, to this oak.