BaSnO_3 (BSO) is a transparent conductive oxide. This category of materials is interesting for applications such as optically transparent electrodes in solar cells and displays. This perovskite-material possesses many interesting properties including a wide bandgap, 3 eV, and a high electrical conductivity (exceeding 10^4 S/cm at room-temperature), which make it very interesting for visible-transparent applications. The DC conductivity in BSO can be superior to that in ITO, which is a commonly used transparent conductive oxide. Thin films used in our study were grown by molecular beam epitaxy (MBE) on LSAT substrates. The epitaxial structure of the samples consist of 45 nm of La-doped BSO on top of a 45 nm thick undoped BSO film grown on LSAT. The BSO films were characterized by means of terahertz spectroscopy. The terahertz-extracted optical conductivity was ~0.8x10^3 S/cm in the 0.1 to 2 THz frequency range. Using these films, upon patterning into stripes, we demonstrate a terahertz polarizer. The polarizer is transparent at visible wavelengths, and functional at terahertz wavelengths; it achieves 96% transmission for terahertz polarization parallel to the stripes and 16% transmission for the perpendicular polarization. Furthermore, we also show that resonant structures, such as cross resonators, are also realizable in this material. The large optical conductivity in BSO films at terahertz frequencies, together with being transparent at visible wavelengths, makes it a very good candidate for developing visible-transparent electromagnetic structures responding in the terahertz frequency range.