Sodium alginate (SA), known as a type of polysaccharide, was one of the main foulants in a water ultrafiltration (UF) system. In this study, internal membrane fouling of rhodamine-labeled SA filtration through polyethersulfone (PES) UF hollow-fiber membranes was directly observed using fluorescent microscopy. Different molecular weight cut-offs (MINCOs) and morphologies of PES membranes resulted in different internal accumulation of SA in membranes. Fluorescent microscope images showed that SA molecules tended to accumulate at the vacant spaces inside membranes without forming a foulant layer on membrane surfaces during filtration. SA that accumulated at the vacant spaces was removed by backwashing. Filtration types (inner-pressured and outer-pressured) also affected internal fouling of SA due to asymmetric structures of PES membranes, regardless of the similar flux and rejection results between the filtration types. Understanding internal fouling provides important insights into membrane fouling and useful information for the rational design of a filtration system using a hollow-fiber membrane.