This study analyses the effectiveness of the biopolymer xanthan gum (XG) to improve the mechanical properties of silty sand. The study focuses on the effects of the curing time and the content of XG on the unconfined and confined behaviour of soil treated with this biopolymer. The study was carried out with specimens cured under high-humidity conditions, through the comparison of the unconfined compressive strength (UCS), stiffness, compressibility and vertical yield stress of non-stabilised and biostabilised specimens, obtained from UCS and oedometer tests. Additionally, the pH value and results of scanning electron microscopy (SEM)/ energy-dispersive X-ray analysis (EDX) tests are presented and discussed. The biostabilisation of this soil with XG induces a more ductile stress–strain behaviour, a moderate increase in UCS and stiffness, and in the increase of the one-dimensional compressibility, with a greater effect for higher XG contents. The SEM/EDX tests show that the biopolymer induces a random biopolymer network in the biostabilised specimens that is related to the strengthening obtained.