Industry is currently in a period of great expansion, the so-called “Industry 4.0”. It relies on the development of new sensor technologies for the generation of systems capable of collecting, distributing and delivering information. Particularly, on Chemical and Biochemical industries, the development of portable monitoring devices can improve many process parameters, like safety and productivity. In this work, the design of a smartphone-based optical fiber sensing platform for the online assessment of fed-batch fermentation systems is reported. The setup is comprised of a smartphone equipped with a 3D-printed case and an application for analyzing the pixel intensity, which is correlated to the broth refraction index (function of sucrose concentration). A sensitivity of 85.83 RIU^-1 (refractive index unit) was verified, and the sensor performance was compared to a handheld refractometer and to model predictions. It showed to be a reliable, portable and low-cost instrument for online monitoring bioreactors, easily reproducible on-site by simply printing it.