Purpose To assess the biomechanical effects of intra-tendinous injections of PRGF on the healing Achilles tendon after repair in a sheep model. Methods Thirty sheep were randomly assigned into one of the six groups depending on the type of treatment received (PRGF or placebo) and survival time (2, 4 and 8 weeks). The Achilles tendon injury was repaired by suturing the tendinous edges employing a three-loop pulley pattern. A trans-articular external fixation system was then used for immobilization. The PRGF or placebo was administered on a weekly basis completing a maximum of three infiltrations. The force, section and tension values were compared between the operated and healthy Achilles tendons across all groups. Results The PRGF-treated tendons had higher force at 8 weeks compared with the placebo group ( p  = 0.007). Between 2 and 4 weeks, a significant increase in force in both the PRGF-treated tendon ( p  = 0.0027) and placebo group ( p  = 0.0095) occurred. No significant differences were found for section ratio between PRGF-treated tendons and the placebo group for any of the time periods evaluated. At 2 weeks, PRGF-treated tendons had higher tension ratio compared with placebo group tendons ( p  = 0.0143). Both PRGF and placebo treatments significantly improved the force ( p  < 0.001 and p  = 0.0095, respectively) and tension ( p  = 0.009 and p  = 0.0039, respectively) ratios at 8 weeks compared with 2 weeks. Conclusion The application of PRGF increases Achilles tendon repair strength at 8 weeks compared with the use of placebo. The use of PRGF does not modify section and tension ratios compared with placebo at 8 weeks. The tension ratio progressively increases between 2 and 8 weeks compared with the placebo.