<p>In recent years, nanotechnology has gained considerable attention in civil engineering as an effective means of enhancing the performance of conventional construction materials. In geotechnical applications, soil–cement is extensively utilized for ground improvement, although its relatively limited strength compared with that of concrete often necessitates performance enhancement. This study examines the effects of nanosilica (NS) on the unconfined compressive strength (UCS) and splitting tensile strength (STS) of soil–cement composites. Cylindrical samples were prepared with varying NS contents of 0%, 1%, 2%, and 3% by weight of dry cement and cured for 7, 14, and 28 days. The results demonstrate that mixtures containing 15% and 20% cement with 2% nanosilica exhibit the most significant improvement in UCS, along with a marked increase in tensile behavior, indicating the positive contribution of nanosilica to the mechanical performance. Microstructural analysis by scanning electron microscopy (SEM) revealed that nanosilica promoted the formation of additional calcium silicate hydrate (C–S–H) and calcium aluminate silicate hydrate (C–A–S–H) gels while reducing the calcium hydroxide (CH) content, thereby producing a denser matrix with a reduced pore structure. This study highlights the role of nanoscale reinforcement in soil–cement systems by correlating the observed strength improvements with microstructural densification. Overall, the findings suggest that nanosilica is a promising additive for enhancing the strength characteristics of soil–cement in geotechnical engineering applications.</p>