Controlling particle size is essential for crystal quality in the chemical and pharmaceutical industry. Several articles illustrate the potential of ultrasound to tune this particle size during the crystallization process. This paper investigates how ultrasound can control the particle size distribution (PSD) of acetaminophen crystals by continuous seed generation in a tubular crystallizer followed by batch growth. It is demonstrated that the supersaturation ratio at which ultrasound starts seed generation has a substantial effect on the final PSD while the applied power is insignificant in the studied conditions. The higher the supersaturation ratio, the smaller the final crystals become up to a supersaturation ratio of 1.56. Furthermore, it was shown that ultrasound can also impact the final PSD when applied during the growth phase. Frequencies of 850 kHz or below reduce the final particle size; the lower the applied frequency, the smaller the crystals become. In conclusion, one could state that ultrasound is able to control the particle size during seed generation and subsequent growth until the final particle size.