tThis study describes a proof-of-concept for a compact real-time surface-enhanced Raman spectroscopy(SERS)-online sensing approach for detection of biofouling in drinking water membrane filtration. In thisstudy we created a custom-designed flow-cell that mimics a cross-flow membrane filtration system. Thisenables one to measure changes in surface-foulants, such as Brevundimonas dimiuta (BD) bacteria and ade-nine, under conditions that are similar to conventional membrane filtration systems. For measurementswe used a common portable Raman-spectrometer with a laboratory Raman-probe in combination with aspecially developed gold nanoparticle (Au NP) SERS-sensing area on filter-membranes. This allowed real-time detection of low concentrations of surface-foulants immediately after inoculation into an ultra-purewater reservoir under pressure-driven filtration conditions. We compared these online results with staticmeasurements from an offline, sample-taking approach, using a confocal Raman-laboratory-microscope.The developed Au NP SERS-sensing-area on the membranes proved to be stable over a long period ofsurface fouling investigations and to suppress the strong interfering Raman-signal originating from thecomposition layer of most filtration membranes.