Platelet-rich plasma injections are used as a treatment for chronic midportion Achilles tendinopathy, but evidence for this treatment is limited. In adults with midportion Achilles tendinopathy, to assess the effects of a single platelet-rich plasma injection, compared with sham injection, on the outcome of the Victorian Institute of Sport Assessment-Achilles (VISA-A) score (a single composite measure of Achilles tendinopathy severity). A participant-blinded, multicenter randomized clinical trial that included 240 people from 24 sites assigned to either a platelet-rich plasma injection or a sham injection between April 2016 and February 2020. Final follow-up was July 2020. Participants were older than 18 years with midportion Achilles tendon pain for more than 3 months as confirmed by ultrasound, magnetic resonance imaging, or both. A single intratendinous platelet-rich plasma injection (n = 121) or a single sham injection (insertion of a subcutaneous dry needle not entering the tendon) (n = 119). The primary outcome was the VISA-A score, measured 6 months after treatment allocation. The VISA-A score contains 8 questions that cover 3 domains of pain, function, and activity, analyzed as a composite score (range, 0 worst symptoms to 100 no symptoms; minimal clinically important difference in score, 12 points). The primary analysis was adjusted for laterality, age, sex, and baseline VISA-A score. Among 240 patients assigned to a platelet-rich plasma or sham injection (mean age, 52 years; 138 58% women), 221 (92%) completed the trial. At 6-month follow-up, mean VISA-A score values in the plasma-rich plasma group vs the sham injection group were 54.4 vs 53.4 (adjusted mean difference, -2.7 95% CI, -8.8 to 3.3). The most common adverse events compared between patients in the platelet-rich plasma group vs the sham group were injection site discomfort (97 vs 73 patients), swelling (56 vs 52 patients) and bruising (48 vs 49 patients). Among patients with chronic midportion Achilles tendinopathy, treatment with a single injection of intratendinous platelet-rich plasma, compared with insertion of a subcutaneous dry needle, did not reduce Achilles tendon dysfunction at 6 months. These findings do not support the use of this treatment for chronic midportion Achilles tendinopathy. isrctn.org Identifier: ISRCTN13254422.