Along with rising concerns about fossil fuel depletion and associated climate change, there are research efforts worldwide to explore renewable and carbon-neutral energy resources. Among various candidates, solar energy stands out as the most attractive candidate because of the gigantic amount that is available. In order to capture and store the intermittent solar power in chemical forms, light-driven water splitting to produce H2 and O2 has been the focus of renewable energy catalysis for decades. This chapter highlights the recent advances of water-splitting catalysis, with a particular emphasis on the development of hybrid molecular–nanomaterial assemblies for H2 and O2 evolution reactions. Hybrid systems based on earth-abundant metals, such as iron, cobalt, and nickel, have been intensively studied for H2 evolution catalysis, while for O2 evolution, ruthenium and iridium catalysts have attracted much attention and the involved mechanistic steps are relatively well elucidated. Recent years have also witnessed the rapid advancement of molecular–nanomaterial assemblies incorporating more inexpensive transition metals, including manganese, iron, and cobalt, for effective O2 evolution catalysis. A diverse array of nanomaterial supports, such as polymer, oligomer, carbon nanotube, graphene, and nanostructured semiconductors, have been employed as catalyst supports. This chapter emphasizes the formation of hybrid catalytic systems with enhanced performance compared to homogeneous molecular counterparts. As clearly shown by many the examples discussed in this chapter, hybrid molecular–nanomaterial assemblies represent a unique category of catalytic systems for water splitting, synergistically integrating the advantages of both homogeneous and heterogeneous catalytic reactions.