Objective evidence regarding the local effects of cupping on muscle and fascial stiffness remains limited. We investigated the acute effects of posterior lower leg cupping on ankle dorsiflexion range of motion (ROM), muscle and fascial stiffness assessed by shear-wave elastography (SWE), and lower-leg muscle activity during passive dorsiflexion. Sixteen healthy university students (10 men and 6 women; age: 20.7 ± 0.8 years; height: 173.4 ± 8.0 cm; body mass: 69.1 ± 9.8 kg) underwent cupping (CUP; moving cupping applied to the posterior lower leg for 1 min) and control (CON; 5 min supine rest) conditions in random order with a minimum 1-week washout period. The shear moduli of the deep fascia, superficial portion of the medial gastrocnemius (UMG) alongside its middle and deep portions, and deep intermuscular fascia were measured using SWE before and immediately after each condition. Ankle dorsiflexion ROM and lower-leg muscle activity during passive dorsiflexion were also assessed. Significant condition × time interactions were observed for ROM (F = 31.10, p < 0.001, η²p = 0.675), deep fascia (F = 10.38, p = 0.006, η²p = 0.409), and UMG (F = 11.73, p = 0.004, η²p = 0.439). Under CUP, ROM increased significantly (+5.65 ± 4.08°, p < 0.001, dz = 1.39), whereas shear moduli of deep fascia (-1.49 ± 1.06 kPa, p < 0.001, dz = -1.41) and UMG decreased significantly (-0.75 ± 0.51 kPa, p < 0.001, dz = -1.46). No significant changes occurred under CON for any of these variables. No significant changes were observed in the middle or deep portions of the medial gastrocnemius, deep intermuscular fascia, or muscle activity. A single bout of posterior lower leg cupping significantly reduced deep fascial and superficial medial gastrocnemius stiffness and increased ankle dorsiflexion ROM without altering muscle activity. These findings suggest that the mechanical effects of suction-based stimulation extend continuously from the skin through the deep fascia to the superficial muscle layer, and that the improvement in ROM is associated with changes in tissue mechanical properties rather than changes in neuromuscular activation.