Objective A prospective, randomized, double-blind, placebo-controlled, parallel-group multicentre study to evaluate the efficacy and safety of diclofenac 4% spray gel for the treatment of acute, uncomplicated ankle sprain. Methods Outpatients with acute, uncomplicated, one-sided ankle sprain were randomly assigned to receive diclofenac 4% spray gel or placebo (vehicle) three times daily for 14 ± 1 days. The main efficacy endpoint was the intra-individual response to treatment (≥50% decrease in swelling of the injured ankle after a treatment period of ≤10 days). Results The response rate was significantly higher in the diclofenac group ( n = 118) than the placebo group ( n = 114) (91.5% vs. 82.5%). After 3–4 days’ treatment, diclofenac spray significantly reduced swelling, spontaneous pain, pain on active movement and tenderness compared with placebo. Diclofenac spray was well tolerated, with a low overall rate of adverse events. Conclusions Diclofenac 4% spray gel rapidly relieves pain and improves mobility in patients with acute, uncomplicated ankle sprain and is well tolerated. It may be a useful treatment option for other acute soft tissue injuries.