Abstract Objectives To investigate the dynamics of response of synovitis to IL-17A inhibition with secukinumab in patients with active PsA using Power Doppler ultrasound. Methods The randomized, placebo-controlled, Phase III ULTIMATE study enrolled PsA patients with active ultrasound synovitis and clinical synovitis and enthesitis having an inadequate response to conventional DMARDs and naïve to biologic DMARDs. Patients were randomly assigned to receive either weekly subcutaneous secukinumab (300 or 150 mg according to the severity of psoriasis) or placebo followed by 4-weekly dosing thereafter. The primary outcome was the mean change in the ultrasound Global EULAR and OMERACT Synovitis Score (GLOESS) from baseline to week 12. Key secondary endpoints included ACR 20 and 50 responses. Results Of the 166 patients enrolled, 97% completed 12 weeks of treatment (secukinumab, 99%; placebo, 95%). The primary end point was met, and the adjusted mean change in GLOESS was higher with secukinumab than placebo −9 (0.9) vs −6 (0.9), difference (95% CI): −3 (−6, −1); one-sided P=0.004 at week 12. The difference in GLOESS between secukinumab and placebo was significant as early as one week after initiation of treatment. All key secondary endpoints were met. No new or unexpected safety findings were reported. Conclusion This unique ultrasound study shows that apart from improving the signs and symptoms of PsA, IL-17A inhibition with secukinumab leads to a rapid and significant reduction of synovitis in PsA patients. Trial registration ClinicalTrials.gov; NCT02662985. Video Abstract 10.1093/rheumatology/keab628_video1 Video Abstract keab628media1 6284681081001