The biosilica shell of marine diatoms has emerged as a unique matrix for photocatalysis, owing to its sophisticated architecture with hierarchical nanopores and large surface area. Although the deposition of titania nanoparticles on diatom biosilica has been demonstrated previously, their photocatalytic activity has been tested only for degradation of pure compounds, such as dyes, nitrogen oxide, and aldehydes. The efficiency of such photocatalysts for degradation of mixtures, for instance, industrial wastewaters, is yet to be investigated. Furthermore, reports on the lattice structures and orientation of nanotitania crystals on biosilica are considerably limited, especially for the underexplored tropical marine diatoms. Here, we report an extensive characterization of titania-loaded biosilica from the tropical Cyclotella striata diatom, starting from freshly grown cell cultures to photodegradation of wastewaters, namely, the palm oil mill effluent (POME). As Indonesia is the largest palm oil producer in the world, photocatalytic technology could serve as a sustainable alternative for local treatment of POME. In this study, we achieved a 54% loading of titania on C. striata TBI strain biosilica, as corroborated by XRF analyses, which was considerably high compared to previous studies. Through visualization using HR-TEM, supported by SAED and XRD analyses, nanocrystal TiO2 appeared to be trapped in an anatase phase with polycrystalline characteristics and distinct crystallographic orientations. Importantly, the presence of C. striata biosilica lowered the band gap of titania from 3.41 eV to around 3.2 eV upon deposition, enabling photodegradation of POME using a broad-range xenon lamp as the light source, mimicking the sunlight. Kinetic analyses revealed that POME degradation using the photocatalysts followed quasi-first-order kinetics, in which the highest titania content resulted in the highest photocatalytic activity (i.e., up to 47% decrease in chemical oxygen demand) and exhibited good photostability throughout the reaction cycles. Unraveling the structure and photoactivity of titania-biosilica catalysts allows transforming marine diatoms into functional materials for wastewater photodegradation.