Pain in the attachment of the plantar fascia in the calcaneus represents 10% of all sports injuries, affects 10% of foot runners, and will affect around 20% of the world population. There is no effective conservative treatment for it. This paper justifies a new definition and name for this pathology, Plantar Fascia Syndrome (PFS), presents a methodology for its diagnosis, and presents the clinical and functional effectiveness of a new conservative treatment based on platelet-rich plasma (PRP). In total, 25 patients (from an initial sample of 260) diagnosed with recalcitrant PFS lasting for more than 12 months were treated with a single infiltration of 2 mL of PRP, according to a new technic proposed. The study was approved by the ethical committee for clinical research of the reference hospital. The patients were controlled after 15, 30, 90, and 180 days, reviewing on each occasion pain, thickness of the plantar fascia, and active extension of the ankle joint. A total of 15 days after infiltration, 85% of patients had no clinical signs requiring treatment. After 90 days of infiltration, no patients showed clinical signs. This improvement in the patients’ condition lasted for 180 days. All patients after treatment can fully resume normal activity with no pain.