This article focuses on understanding the role of vital wheat gluten on the structural parameters of extruded fish feed and its correlation to the physical and functional properties. Gluten–soy protein concentrate blends with five gluten concentrations (0–200 g kg−1) were produced. An abrupt reduction in oil uptake was observed with the 200 g gluten kg−1 blend. Inclusion of gluten from 100 to 200 g kg−1 resulted in unacceptable product properties. Sinking of feed pellets with 0 and 50 g gluten kg−1 was 100%, whereas only 36% of pellets with 200 g gluten kg−1 sank. We suspect that this is due to a relationship between morphological structure and oil impregnation during coating of feeds. The addition of gluten at 200 g kg−1 gave a smoother and non-porous outer surface. Pellets without gluten had a larger number of cells that were smaller than 200 μm (P < 0.05) compared with pellets with 100 and 200 g gluten kg−1. More spherical cell shapes (P < 0.01) and a compact structure were favoured in the presence of gluten. The closed porosity increased (P < 0.05), whereas interconnectivity between pores decreased (P < 0.01), with increasing gluten content from 0 to 200 g kg−1. The effects of the addition of gluten are probably related to the film-forming properties of gluten.