Trenching caused by mooring chains significantly reduces the bearing capacity of suction anchors in offshore mooring systems. The unloading effects associated with trench formation lead to stress redistribution in the surrounding soil, altering parameters such as shear strength and consequently affecting the anchor’s ultimate capacity. This study employs finite element modeling to evaluate trench-induced unloading effects. By modifying the soil parameters of trenched seabeds according to pre- and post-trenching stress states, the differences in bearing capacity are quantified. The results show that unloading effects reduce the strength of the remaining soil, with pronounced sensitivity to the thickness of the wall-attached soil layer between the anchor sidewall and the trench, the anchor aspect ratio, and the trench width. Moreover, regardless of whether unloading effects are considered, the wall-attached soil layer substantially influences the ultimate horizontal bearing capacity of suction anchors in trenched seabeds, contributing more than 10% of the total capacity. Finally, modification factor formulas—defined as the ratio of ultimate capacities with versus without unloading effects—are developed from numerical results. These formulas enable corrected capacity predictions for trenched seabeds where unloading effects were initially disregarded.