Most guidelines recommend against PSA-based screening for prostate cancer in men ≥ 70 years of age. Adherence to these guidelines is variable. To determine whether the use of a “Best Practice Advisory” (BPA) intervention within the electronic medical record (EMR) system can alter the rate of PSA screening in men ≥ 70 years of age. This is an interventional study spanning the years 2013 through 2017, in men ≥ 70 years of age in Kaiser Permanente Northern California with no prior history of prostate cancer. The BPA intervention was activated in the EMR system on October 15, 2015, with no prior notice or education. Integrated healthcare system including all Kaiser Permanente Northern California facilities. A population-based sample that included all male members ≥ 70 years of age without a history of prostate cancer. The main outcome was the rate of PSA testing in men ≥ 70 years of age. We compared the rates of PSA testing between the pre-BPA period (January 1, 2013-October 14, 2015) and the post-BPA period (October 15, 2015-December 31, 2017). An interrupted time series analysis of PSA ordering rates was performed. Following the 2015 BPA intervention, screening rates substantially declined from 36.0 per 100 person-years to 14.9 per 100 person-years (rate ratio = 0.415; 95% CI: 0.410-0.419). The effect of the BPA was comparable among all patient races and ordering provider specialties. The interrupted time series analysis showed a rapid, large, and sustained drop in the rate of PSA ordering, and much less temporal variation in test ordering after activation of the BPA. Following activation of a BPA within the EMR, the rates of inappropriate PSA testing significantly declined by 58.5% in men ≥ 70 years of age and temporal variation was reduced.