Patellar tendinopathy is a common injury in athletes. In this study we wanted to examine whether 1 injection of platelet-rich plasma (PRP) was more effective than placebo (saline) in the treatment of athletes with patellar tendinopathy. Twenty-four athletes with patellar tendinopathy of at least 6 months’ duration were randomly assigned to receive either a blinded injection of PRP (n = 12) or saline (n = 12). The primary endpoint was improvement in Victorian Institute of Sports Assessment-Patella (VISA-P) score at 3 months. Secondary outcomes included changes in pain at rest, while walking, and when the tendon was squeezed. Ultrasonographic outcomes were changes in tendon thickness, the presence and size of a central hypoechoic area, and color Doppler activity. There was no difference between the PRP group and the saline group with regard to the primary outcome, VISA-P score mean difference 5.4 (95% confidence interval –5.5 to 16.4, P = 0.316). There were no statistically significant differences observed in any of the secondary outcomes. In conclusion, in this blinded, randomized, controlled trial, there was no improvement in VISA-P score, ultrasonography, or any other outcome measures 3 months after an injection of PRP compared with a saline injection.