Mixed-dimensional van der Waals heterostructures combine the advantages of nanomaterials with qualitatively distinct properties such as the extended bandstructures and high charge carrier mobilities of inorganic two-dimensional materials and the discrete orbital energy levels and strong optical absorption of zero-dimensional organic molecules. The synergistic interplay between nanomaterials of distinct dimensionality has enabled a variety of unique applications such as antiambipolar transistors, sensitized photodetectors, and gate-tunable photovoltaics. Because the performance of mixed-dimensional heterostructure devices depends sensitively on the buried interfacial structure, it is of great interest to identify materials and chemistries that naturally form highly ordered heterointerfaces. Toward this end, here we demonstrate ultrahigh vacuum self-assembly of 2,7-dioctylbenzothieno [3,2-b]benzothiophene (C8-BTBT) monolayers onto epitaxial MoS2/graphene heterostructures. With molecular-resolution scanning tunneling microscopy and spectroscopy, the resulting C8-BTBT/MoS2/graphene mixed-dimensional heterostructures are found to be rotationally commensurate with well-defined physical and electronic structures. It is further shown that the self-assembled C8-BTBT monolayers are insensitive to the structural defects and electronic perturbations of the underlying MoS2 substrate, which provides significant processing latitude. For these reasons, this work will facilitate ongoing efforts to utilize organic/MoS2/graphene mixed-dimensional heterostructures for electronic, optoelectronic, and photovoltaic applications.