Defect reduction in epitaxial InP on nanopatterned exact Si (001) substrates was investigated. Top-down lithography and dry etching were used to define 30 nm-wide SiO trench openings, with concaves recessed into the Si substrates. Uniformly distributed and position-controlled InP seed arrays were formed by selective area growth. Afterwards, the SiO mask was removed and InP overgrowth on the seed arrays proceeded. By localizing defects in the buried Si concaves and promoting defect interactions during the coalescence process, a significant reduction in the x-ray linewidth has been achieved for InP layers grown on the nanopatterned Si as compared to blanket epitaxy. Anisotropic defect distribution in the coalesced InP films was observed and its dependency on seed layer thickness was also studied.