Bug, the name of two rivers of eastern Europe. The Southern Bug is in Ukraine. It flows to the southeast into the Black Sea near Nikolayev. It is about 530 miles (850 km) long. The Western Bug, which begins in Ukraine, forms part of the boundaries between Poland and Ukraine and between Poland and Belarus. The river flows north for about 200 miles (320 km) before turning westward to enter the Vistula River north of Warsaw. It is navigable for about half of its 500-mile (800-km) course.