Background: Rupture of the common calcaneal tendon is the second most frequent tendon rupture in dogs and may lead to severe lameness and pain. Surgical repair consists of re-apposition of the damaged tendon ends using sutures, but this type of repair is not always possible especially if the tendon has retracted. Tendon augmentation with an ultra-high molecular weight polyethylene (UHMWPE) implant is a recent solution to support the sutures and allow the repair of the canine calcaneal tendon. However, its biomechanical fixation strength remains untested for this pathology. Aim: To evaluate the biomechanical fixation strength of a UHMWPE implant for the repair of the canine calcaneal tendon. Methods: Ex-vivo biomechanical study was carried out on eight cadaveric hindlimbs from four adult dogs. Hindlimbs were tested under two independent modalities: proximal tendinous fixation (PTF) and distal calcaneus fixation (DCF), using a testing machine. PTF was achieved by eight simple interrupted polypropylene sutures performed through the UHMWPE implant. The latter was sandwiched inside the gastrocnemius tendon, which had previously been incised over about 5 cm longitudinally, and through the tendon of the superficial digital flexor. DCF was performed using an interference screw, which locked the UHMWPE implant into a calcaneus tunnel drilled perpendicularly. Results: Yield, failure load, and linear stiffness (mean ± SD) for the DCF modality were 920 ± 139 N, 1,007 ± 146 N, and 92 ± 15.21, respectively, which were greater than for the PTF modality (663 ± 92 N, 685 ± 84 N and 25.71 ± 5.74, respectively, p < 0.05). Failure modes were different between fixation modalities: for PTF it was suture breakage ( n = 7/8), while for DCF it was implant damage and slippage ( n = 8/8). Conclusion: The biomechanical fixation strength of the UHMWPE implant was greater for DCF than that of PTF, and should be suitable for calcaneal tendon repair in dogs. The clinical prediction of rupture of this calcaneal tendon repair will occur at the level of the PTF.