<p>This study presents a novel closed-form analytical framework for predicting the ultimate tensile capacity of helical anchors in unsaturated clay. It addresses a significant gap in current design methodologies that predominantly consider saturated soil conditions. The proposed model explicitly incorporates the mechanics of unsaturated soils by integrating matric suction effects on shear strength mobilization via Bishop’s effective stress principle and the soil–water retention curve. The formulation accounts for two primary failure mechanisms: cylindrical shear for multi-helix anchors with close spacing (S/D ≤ 1.5) and individual conical failure for single or widely spaced helices. For each mechanism, the solution rigorously considers the contributions of soil weight, shear, and normal forces acting on the failure surface, as well as shaft friction, and incorporates modified lateral earth pressure coefficients for unsaturated states. The analytical solution was validated against a comprehensive set of laboratory pullout tests on instrumented helical anchors in unsaturated clay. The model demonstrated excellent predictive capability, with a high coefficient of determination (R² = 0.98) and a mean relative error of 10.8%. A subsequent parametric study elucidated the profound influence of key variables, revealing that pullout capacity increases with embedment depth, helix diameter, and reduced helix spacing. Notably, the analysis identified a non-monotonic relationship with moisture content, showing that capacity peaks at an intermediate water content (~ 7% for the tested soil), corresponding to the maximum suction stress. It is shown that the increase in pull-out capacity can reach 70% of the saturated case, depending on the helix diameter and spacing ratio.</p>