We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N,N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide(PTCDI-C-8) by surface X-ray crystallography. We show that PTCDI-C-8 forms smooth layered films on Al2O3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 +/- 2 degrees with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.