The crystal structure of Na 0 . 5 Bi 2 . 5 Nb 2 O 9 was refined from its powder X-ray diffraction (XRD) data. Sodium bismuth niobate at 23 o C is orthorhombic, A2 1 am, with a=5.4998(3)Å, b=5.4602(2)Å, c=24.952(1)Å, and Z=4. The piezoelectric properties were investigated using the dense bulk ceramics. The electromechanical coupling coefficients (k i j ) and electrical quality factors (Q m ) are k 3 1 =3.2%, k t =10.0%, and Q m =3800. Single crystals were also grown from stoichiometric melts using a slow cooling technique. The dielectric and ferroelectric properties of the crystal indicates that a spontaneous polarization clearly occurs in the crystallographic a-b plane.