We developed a facile interfacial oriented growth and self-assembly process to fabricate three-dimensional (3D) aligned mesoporous iron oxide nanopyramid arrays (NPAs). The unique NPAs possess a 3D mesostructure with multiple features, including high surface area (∼175 m2/g), large pore size (∼20 nm), excellent flexibility (bent over 150 times), and scalability at the foot scale for practical applications. More importantly, these NPAs structures enable versatile enhancement of localized surface plasmon resonance and photoelectrochemical conversion. The integration of plasmonic gold with 3D NPAs remarkably improves the performance of photoelectrochemical conversion, leading to ∼6-and 83-fold increases of the photocurrent under simulated solar and visible-light illumination, respectively. The fabrication and investigation of NPAs provide a new paradigm for preparing unconventional mesoporous oriented thin films and further suggest a new strategy for designing plasmonic metal/semiconductor systems for effective solar energy harvesting.