Sian Lowland or Oversian basin (Ukrainian: Надсіанська котловина) is a lowland located at the border between Poland and Ukraine along the San River. Most of the region is located in Poland except for a small portion in southeastern end which is in Ukraine.
A large proportion of the lowland's forests have been cleared out, although pine forests mixed with firs and birches occur in the region's sandy areas, and fir forests mixed with hornbeams and maples grow in its heavier soils.
The lowland has a population density of 80 persons/km2; the population is predominantly (75%) rural.