<p>The use of biopolymers for sand stabilization has garnered considerable attention in geotechnical and geoenvironmental applications. Biopolymers improve the stability and strength characteristics of treated sand; however, managing retention capacity and hydraulic conductivity remains crucial. These parameters require a special design approach to couple mechanical and hydraulic performance effectively, ensuring precise control over the flow of water or contaminants through biopolymer-treated layers. The soil–water retention curve (SWRC) serves as a critical tool for bridging this gap, facilitating the application of unsaturated soil mechanics to the advanced modeling and design of such systems. This study investigates the influence of fine content on the SWRC and retention capacity of sand treated with sodium alginate and guar gum biopolymers at dosages of 1, 3, and 5%. Fine content was incorporated at levels of 10, 20, and 30% for each biopolymer dosage. The SWRC was determined using a combination of axis translation technique and vapor equilibrium technique, enabling coverage of the full suction range. The findings revealed a dual retention trend across low to moderate suction ranges, with significant enhancement observed at high suction levels. At low and moderate suctions, retention capacity was primarily governed by macropores, which were influenced by the interaction between biopolymers, clay minerals, and fine contents that partially or completely filled these pores. The degree of alteration in retention capacity was directly proportional to the biopolymer dosage, with more pronounced effects observed at lower biopolymer concentrations.</p>