This is a pioneering study on the synthesis and application of composites based on micro- and nanodiamonds for the photocatalytic degradation of environmental water pollutants. Micro- and nanodiamond powders (with particle sizes of 1–3 μm and 2–10 nm, respectively) were combined with TiO2, by varying the carbon-phase content, and tested as composite photocatalysts for the degradation of diphenhydramine, which is a pharmaceutical water pollutant, under near-UV/Vis irradiation. These composites exhibited higher photocatalytic activity than the respective bare materials. In addition, composites prepared with pristine nanodiamonds were always more active than those prepared with microdiamonds of the same carbon content. A significant enhancement in the photocatalytic performance was observed on preparation of the composite with 15 wt % of nanodiamonds oxidised in air at 703 K; these oxidised nanodiamonds contained mainly carboxylic anhydrides, lactones, phenols and, to a lesser extent, carbonyl/quinone groups on their surface.