To investigate the effectiveness of ultrasound guided corticosteroid injection in the treatment of plantar fasciitis. Randomised, investigator and participant blinded, placebo controlled trial. University clinic in Melbourne, Australia. 82 people with a clinical and ultrasound diagnosis of plantar fasciitis unrelated to systemic inflammatory disease. Participants were randomly allocated to ultrasound guided injection of the plantar fascia with either 1 mL of 4 mg/mL dexamethasone sodium phosphate (experimental group) or 1 mL normal saline (placebo). Before injection the participants were given an ultrasound guided posterior tibial nerve block with 2% lidocaine (lignocaine). Primary outcomes were pain, as measured by the foot health status questionnaire (0-100 point scale), and plantar fascia thickness, measured by ultrasound at 4, 8, and 12 weeks. Reduction in pain at four weeks favoured the dexamethasone group by 10.9 points (95% confidence interval 1.4 to 20.4, P=0.03). Between group differences for pain scores at eight and 12 weeks were not statistically significant. Plantar fascia thickness measured at four weeks favoured the dexamethasone group by -0.35 mm (95% confidence interval -0.67 to -0.03, P=0.03). At eight and 12 weeks, between group differences for plantar fascia thickness also favoured dexamethasone, at -0.39 mm (-0.73 to -0.05, P=0.02) and -0.43 mm (-0.85 to -0.01, P=0.04), respectively. The number needed to treat with dexamethasone for one successful outcome for pain at four weeks was 2.93 (95% confidence interval 2.76 to 3.12). There were no reported adverse events associated with the intervention. A single ultrasound guided dexamethasone injection is a safe and effective short term treatment for plantar fasciitis. It provides greater pain relief than placebo at four weeks and reduces abnormal swelling of the plantar fascia for up to three months. However, clinicians offering this treatment should also note that significant pain relief did not continue beyond four weeks. Australian New Zealand Clinical Trials Registry ACTRN12610000239066.