Abstract Floating Production Storage and Offloading Units (FPSOs) will continue to be in high demand because of their numerous advantages; such as, their ability to offer early production and operate in ultra-deep water locations, while remaining easy to relocate to meet changing needs. By design, slack cargo tanks are almost always present in FPSOs due to continuous loading and offloading operations; however, the presence of slack cargo impacts the seakeeping stability abilities of FPSOs. There are limited published experimental data on coupled sloshing with seakeeping of stationary vessels, and existing studies on this have focused on single row cargo tanks which is seldom the case in FPSOs. The aim of this paper is to study roll motion coupled with sloshing in partially-filled pair of two-row tanks of a stationary FPSO model exposed to regular beam waves. The model tests was performed in the Davidson Laboratory towing tank at Stevens Institute of Technology. The FPSO model response in roll was measured, and the time histories of sloshing oscillation were measured on the starboard and port side of one of the stern cargo tanks. The results show that varying internal cargo sloshing leads to unpredictable motion response of floating vessels that should be accounted for in the design and offloading operations of FPSO.