Organometallic nanocomposites represent a frontier in the development of multifunctional materials for energy conversion and environmental remediation. These materials integrate the tunable electronic and structural properties of organometallic compounds with the unique physicochemical characteristics of nanoscale supports, generating synergistic effects that surpass conventional catalytic and functional materials. The rational design of organometallic nanocomposites encompasses the precise selection of metal centers, ligands, and host matrices to achieve enhanced stability, electron transfer efficiency, and surface reactivity. Synthetic strategies, ranging from bottom-up self-assembly to advanced deposition techniques, allow for controlled size, morphology, and interfacial interactions, critical factors for performance optimization. Applications in energy conversion include photocatalytic water splitting, electrocatalysis for hydrogen evolution, and integration into photovoltaic systems, where these nanocomposites exhibit high activity, selectivity, and long-term durability. In environmental remediation, organometallic nanocomposites demonstrate exceptional capabilities in pollutant degradation, heavy metal sequestration, and greenhouse gas mitigation. This chapter provides a comprehensive overview of the current state-of-the-art in the design, synthesis, characterization, and applications of organometallic nanocomposites, emphasizing mechanistic insights and structure–property relationships that inform next-generation material development. Challenges and future perspectives in scalability, environmental compatibility, and integration into real-world devices are also critically analyzed.