Fused filament fabrication (FFF) is the most popular additive manufacturing method, which allows the production of highly complex three-dimensional parts with minimal material waste. On the other hand, polyethylene terephthalate glycol (PETG) has been used to replace traditional polymers for 3D printing due to its chemical resistance and mechanical performance, among other benefits. However, when fibres are added, these PETG-based composites can be suitable for many different applications. Nevertheless, to guarantee their good performance in-service in these applications, and even extend to new ones, it is necessary for their mechanical properties to be maximized. Therefore, this study intends to optimize the printing parameters (nozzle temperature, printing speed, layer height and filling) in order to maximize the mechanical properties of printed PETG, PETG+CF (carbon fibre-reinforced PETG composites) and PETG+KF (aramid fibre-reinforced PETG composites). The Taguchi method was used for the experimental procedure design, and the specimens were produced according to the L16 orthogonal array. Finally, an analysis of variance (ANOVA) was performed, with a 95% confidence interval, to analyse the effect of the printing parameters on the bending properties. It was possible to conclude that the best bending properties for PETG, PETG+CF and PETG+KF were obtained for extrusion temperatures of 265 °C, 195 °C and 265 °C, printing speeds of 20, 60 and 20 mm/s, layer heights of 0.4, 0.53 and 0.35 mm and an infill density of 100% for the three materials, respectively.