Geotextile tubes have offered a cost-effective and convenient solution for the treatment of dredged slurry in recent years. Despite their benefits, the dewatering process of this method remains cumbersome and time-consuming. The incorporation of flocculants into dredged slurries has been shown to mitigate the clogging of geotextile tubes, thereby enhancing the efficiency of the dewatering process. To quantify the impact of flocculant addition on the dewatering performance, a series of laboratory tests were conducted to investigate the sedimentation behavior of flocculant-treated river-dredged slurries under filtration of woven geotextiles. The impact of different flocculants is evaluated with the sedimentation rates of dredged slurries, specifically nonionic polyacrylamide (NPAM) and cationic polyacrylamide (CPAM). Results demonstrated that both NPAM and CPAM, at an optimal concentration of around 250 mg/L, could substantially increase the settling rates of slurries when filtered through geotextiles. At equivalent flocculant concentration, NPAM outperforms CPAM in accelerating sedimentation rates. It is also evidenced that adding flocculants can significantly augment the particle size and permeability of slurries. After adding flocculants, the water content and dry density of the final sediments exhibit a more uniform distribution in the vertical direction compared to the original slurry. Furthermore, scanning electron microscopy was employed to analyze the micromorphology of the final sediments treated with different NPAM concentrations. The results showed that the slurries exhibited significant void structure under treatment with the optimal concentration of NPAM, demonstrating the flocculants’ effectiveness in enhancing the dewatering process of river-dredged slurries when filtered with geotextiles.