In this work, a novel catalytic system for the sunlight-driven water splitting reaction, which exploits the photocatalytic ability of BiVO4 coupled to a new kind of noble-metal-free organic dye molecules, is proposed. Hence, mono- and di-substituted naphthalenediimides (NDIs) were designed to have different functional groups that provide to them both tunable optical properties and adjustable HOMO/LUMO levels, and were selectively prepared (starting from 1,4,5,8-naphthalenetetracarboxylic acid) achieving yields >69%. Smart anchoring groups (i.e. carboxylates or aromatic amines) were added to the dyes in order to allow them to covalently bond to acidic -OH groups present on the BiVO4 surface. An easy and low-cost room temperature dip-coating technique was used to dye-sensitize both BiVO4 powders and thin films. NMR, MS, FT-IR, TG, FESEM, XRD, XPS and optical analyses confirmed the successful organic synthetic routes and good dyes/BiVO4 linkages. Photochemical and photoelectrochemical water oxidation reaction tests, together with DFT calculations, demonstrated that a proper alignment of the semiconductor/NDI-based dye energy levels is fundamental for enhancing the photocatalyst performance through a Z-scheme mechanism. The ability of the NDI organic molecules to delocalize the electronic charges was also a key factor for minimizing recombination processes and achieving more than a ten-fold increase in the photocurrent density of a 6 cm2 BiVO4 photo-electrode. The here reported results open new perspectives for the utilization of this new series of core-substituted NDIs, which are able to improve the activity of photocatalysts for different sunlight-driven applications, e.g. waste water treatment and organic contaminants' degradation, other than the production of solar fuels by water splitting mechanisms.