Background: An acute Achilles tendon rupture (ATR) is a long-lasting and devastating injury. Possible biological augmentation to promote and strengthen tendon healing after an ATR would be desirable. Purpose: To determine whether the application of a platelet-rich plasma (PRP) injection in nonsurgically treated ATRs may promote healing and thereby improve functional outcomes. Study Design: Randomized controlled trial; Level of evidence, 2. Methods: A total of 40 men (aged 18-60 years) with an ATR incurred within 72 hours were included, and 38 were followed for 12 months. All patients were treated with an orthosis with 3 wedges for 8 weeks; full weightbearing from day 1 was allowed, combined with either 4 PRP or 4 placebo injections (a few drops of saline, <0.5 mL, under the skin) 14 days apart. All patients received the same instructions on an exercise program starting from week 9. Outcomes included the self-reported Achilles tendon Total Rupture Score (ATRS) as well as heel-rise work, heel-rise height, tendon elongation, calf circumference, and ankle dorsiflexion range of motion. Results: The mean ATRS score improved in both groups at all time points (P <.001), but there was no difference between the groups at any time points (12 months: 90.1 points in PRP group and 88.8 points in placebo group). No differences in all functional outcomes at any time points were seen between the groups. At 12 months, the injured leg did not reach normal functional values compared with the uninjured leg. Conclusion: The application of PRP in nonsurgically treated ATRs did not appear to show any superior clinical and functional improvement. Registration: NCT02417922 (ClinicalTrials.gov identifier)