We design, fabricate, and experimentally demonstrate an optically active metasurface of λ/50 thickness that rotates linearly polarized light by 45° over a broadband wavelength range in the near IR region. The rotation is achieved through the use of a planar array of plasmonic nanoantennas, which generates a fixed phase-shift between the left circular polarized and right circular polarized components of the incident light. Our approach is built on a new supercell metasurface design methodology: by judiciously designing the location and orientation of individual antennas in the structural supercells, we achieve an effective chiral metasurface through a collective operation of nonchiral antennas. This approach simplifies the overall structure when compared to designs with chiral antennas and also enables a chiral effect which quantitatively depends solely on the supercell geometry. This allows for greater tolerance against fabrication and temperature effects.