Antibiotic stewardship is key to optimizing patient outcomes and affordable care. The study objective was to examine the effect of provider education and clinical decision support (CDS) on antibiotic prescribing for acute sinusitis among providers of varying experience.A stepped-wedge cluster randomized intervention to evaluate antibiotic use for acute sinusitis encounters at 126 Kaiser Permanente Southern California clinics between September 2014 and April 2015.The primary outcome was receipt of an antibiotic prescription. Multivariate analysis adjusted for patient, provider, and medical center characteristics. Secondary analyses described sinusitis and other common upper respiratory infection (URI) diagnoses and antibiotic use during the study period compared with prior years. Chart review of a random sample reported the proportion of encounters receiving guideline-concordant antibiotics.Analysis of 21,949 encounters (10,491 pre- and 11,458 post intervention) showed CDS reduced the use of antibiotics (adjusted odds ratio [AOR], 0.78; 95% CI, 0.71-0.87), although the pre-post absolute difference was small (85.9% vs 83.9%, respectively). Education had a large initial effect (AOR, 0.51; 95% CI, 0.46-0.57), which did not persist. Increasing years of provider experience raised the rates of antibiotic prescribing, but did not have a significant interaction with CDS (P = .19). The effect of CDS varied by medical center (P <.001). In addition, sinusitis diagnoses decreased post intervention, with no overall increase in antibiotic prescribing for URI diagnoses. Lastly, guideline-concordant antibiotic use increased by 14%.Provider education and CDS improved antibiotic stewardship and changed diagnosis patterns. The benefits of education were brief, and CDS effectiveness varied by medical center.