A fuzzy logic control approach is developed for the control of a seeded semibatch crystallizer. Crystallization of paracetamol (PA) in 2-propanol-water mixtures was used as the model system. The concentration and cord length counts were measured using an in situ attenuated total reflection Fourier transform infrared probe and an in situ focused beam reflectance measurement probe, respectively. Three open-loop feeding policies, concave (CCFP, similar to natural cooling), linear (LFP), and convex (CVFP, near controlled cooling), were employed to investigate the process dynamic behavior to construct the fuzzy controller structure for the control of supersaturation within a predefined zone close to the solubility curve. It is found that the fuzzy controller can ensure tracking of the concentration within the zone, leading to substantial improvement of the end product size distribution compared to the open-loop results. Selecting the initial PA concentration above the upper limit of the concentration results in longer process times. The open-loop results show that the feeding policy (addition of water) does not prevent nucleation and agglomeration; however, both phenomena can be minimized by the LFP.