Van der Waals (vdW) semiconductors are promising candidates for next-generation electronic devices. Although plasticity has been observed in these materials, strain hardening and large uniform tensile elongation remain elusive. Here we report that GaSe single crystals exhibit exceptional tensile ductility when loaded along directions inclined to the [0001] zone axis, achieving uniform tensile elongation exceeding 40% together with pronounced strain hardening. Using atomic-resolution, stress-quantified experiments, we uncover a delocalized interlayer shear mechanism in which alternating slip between adjacent vdW layers homogenizes tensile strain and suppresses localization. This cooperative slip process drives an ε-to-γ phase transformation and introduces constrained slip pathways, giving rise to a previously unrecognized vdW strain hardening mechanism. Comparable tensile ductility and strain hardening behaviour are further observed in other chalcogenides, such as InSe and SnSe2, suggesting the generality of this mechanism. These findings revise the mechanical paradigm of vdW semiconductors and establish a basis for their use in flexible and stretchable electronics.