Sloshing is the movement of fluid in a partially filled vessel subjected to external motions. Although sloshing causes intense forces and instability to floating structures, the internal motion of liquid caused by external forces might be useful for devices that require mixing. For offshore floating photobioreactors, sloshing induced by external ocean wave forces are desirable, as the movement of liquid is able to bring about mixing and mass transfer for the nutrients and gas in the microalgae culture. The objective of this study is to investigate the correlation between the regular wave-induced sloshing of different motions (surge, heave, and pitch) on the mixing behaviour of the novel offshore floating photobioreactor designed to utilize the ocean surface waves for the effects of mixing. To determine the residence time distribution (RTD) of the stagnant volume of the photobioreactor, dissolved solids were injected into the system. The concentration of dissolved solids was measured using a total dissolve solid (TDS) sensor. The mixing caused by sloshing motion had the highest intensity during pitching motion. A homogenous solution was formed just after 65.9 s for the filling ratio of 30 , with the rotation angle of 8 degrees and frequency of 0.21 Hz. Thus, wave-induced sloshing is able to generate intense mixing to the medium during pitching motion but still has non-ideal behaviours, such as bypass and dead zones during horizontal and vertical motion. © 2023 Institution of Chemical Engineers