Actinide-based metal−organic complexes and coordination architectures encompass intriguing properties and functionalities but are still largely unexplored on surfaces. We introduce the in situ synthesis of actinide tetrapyrrole complexes under ultrahighvacuum conditions, on both a metallic support and a 2D material. Specifically, exposure of a tetraphenylporphyrin (TPP) multilayer to an elemental beam of thorium followed by a temperatureprogrammed reaction and desorption of surplus molecules yields bis(porphyrinato)thorium (Th(TPP) 2 ) assemblies on Ag(111) and hexagonal boron nitride/Cu(111). A multimethod characterization including X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption, and complementary density functional theory modeling provides insights into conformational and electronic properties. Supramolecular assemblies of Th(TPP) 2 as well as individual double-deckers are addressed with submolecular precision, e.g., demonstrating the reversible rotation of the top porphyrin in Th(TPP) 2 by molecular manipulation. Our findings thus demonstrate prospects for actinide-based functional nanoarchitectures.