Microplastics (MPs) are emerging contaminants that are abundantly present in the influent and effluent of wastewater treatment plants (WWTPs). Forward osmosis (FO) is an advanced treatment technology with po-tential applications in WWTPs. The presence of MPs in WWTP effluents can contribute to FO fouling and per-formance deterioration. This study focuses on FO membrane fouling by MPs of different sizes, and the interactional impacts of MPs and Humic acid (HA) (as the most common organic foulant in WWTPs) on FO membrane performance. The synergistic effect of combined MPs and HA fouling is shown to cause higher flux decline for FO membranes than that of HA or MPs alone. Reverse salt flux increased in the presence of MPs, and decreased when HA was present. Further, full flux recovery was obtained for all fouled membranes after hy-draulic cleaning. This indicates the efficiency of FO systems for treating wastewater with high fouling potential. This study highlights the necessity of considering MPs in studying fouling behaviour, and for mitigation stra-tegies of membranes used in WWT. The fundamentals created here can be further extended to other membrane -assisted separation processes.