© 2018 The Authors Ophthalmic & Physiological Optics © 2018 The College of Optometrists Purpose: Myopia progression is thought to involve biomechanical weakening of the sclera, which leads to irreversible deformations and axial elongation of the eye. Scleral crosslinking has been proposed as a potential treatment option for myopia control by strengthening the mechanically weakened sclera. The biomechanical mechanism by which the sclera weakens during myopia and strengthens after crosslinking is not fully understood. Here, we assess the effect of lens-induced myopia and exogenous crosslinking using genipin on the inelastic mechanical properties of the tree shrew sclera measured by cyclic tensile tests. Methods: Cyclic tensile tests were performed on 2-mm wide scleral strips at physiological loading conditions (50 cycles, 0–3.3 g, 30 s cycle−1). Two scleral strips were obtained from each eye of juvenile tree shrews exposed to two different visual conditions: normal and 4 days of monocular −5 D lens wear to accelerate scleral remodelling and induce myopia. Scleral strips were mechanically tested at three alternative conditions: immediately after enucleation; after incubation in phosphate buffered saline (PBS) for 24 h at 37°C; and after incubation for 24 h in PBS supplemented with genipin at a low cytotoxicity concentration (0.25 mm). Cyclic softening was defined as the incremental strain increase from one cycle to the next. Results: −5D lens treatment significantly increased the cyclic softening response of the sclera when compared to contralateral control eyes (0.10% ± 0.029%, mean ± standard error, P = 0.037). Exogenous crosslinking of the lens treated sclera significantly decreased the cyclic softening response (−0.12% ± 0.014%, P = 2.2 × 10−5). Contrary to all other groups, the genipin-cross-linked tissue did not exhibit cyclic softening significantly different from zero within the 50-cycle test. Conclusions: Results indicated that cyclic tensile loading leads to an inelastic, cyclic softening of the juvenile tree shrew sclera. The softening rate increased during lens-induced myopia and was diminished after genipin crosslinking. This finding suggests that axial elongation in myopia may involve a biomechanical weakening mechanism that increased the cyclic softening response of the sclera, which was inhibited by scleral crosslinking using genipin.