Low-value diagnostic tests have been included on primary care specialty societies' "Choosing Wisely" Top Five lists. To evaluate the effectiveness of a standardized patient (SP)-based intervention designed to enhance primary care physician (PCP) patient-centeredness and skill in handling patient requests for low-value diagnostic tests. Randomized clinical trial of 61 general internal medicine or family medicine residents at 2 residency-affiliated primary care clinics at an academic medical center in California. Two simulated visits with SP instructors portraying patients requesting inappropriate spinal magnetic resonance imaging for low back pain or screening dual-energy x-ray absorptiometry. The SP instructors provided personalized feedback to residents regarding use of 6 patient-centered techniques to address patient concerns without ordering low-value tests. Control group physicians received SP visits without feedback and were emailed relevant clinical guidelines. The primary outcome was whether resident PCPs ordered SP-requested low-value tests during up to 3 unannounced SP clinic visits over 3 to 12 months follow-up, with patients requesting spinal magnetic resonance imaging, screening dual-energy x-ray absorptiometry, or headache neuroimaging. Secondary outcomes included PCP patient-centeredness and use of targeted techniques (both coded from visit audiorecordings), and SP satisfaction with the visit (0-10 scale). Of 61 randomized resident PCPs (31 control group and 30 intervention group), 59 had encounters with 155 SPs during follow-up. Compared with control PCPs, intervention PCPs had similar patient-centeredness (Measure of Patient-Centered Communication, 43.9 [95% CI, 42.0 to 45.7] vs 43.7 [95% CI, 41.8 to 45.6], adjusted mean difference, -0.2 [95% CI, -2.9 to 2.5]; P = .90) and used a similar number of targeted techniques (5.4 [95% CI, 4.9 to 5.8] vs 5.4 [95% CI, 4.9 to 5.8] on a 0-9 scale, adjusted mean difference, 0 [95% CI, -0.7 to 0.6]; P = .96). Residents ordered low-value tests in 41 SP encounters (26.5% [95% CI, 19.7%-34.1%]) with no significant difference in the odds of test ordering in intervention PCPs relative to control group PCPs (adjusted odds ratio, 1.07 [95% CI, 0.49-2.32]). Rates of test ordering among intervention and control PCPs were similar for all 3 SP cases. The SPs rated visit satisfaction higher among intervention than control PCPs (8.5 [95% CI, 8.1-8.8] vs 7.8 [95% CI, 7.5-8.2], adjusted mean difference, 0.6 [95% CI, 0.1-1.1]). An SP-based intervention did not improve the patient-centeredness of SP encounters, use of targeted interactional techniques, or rates of low-value test ordering, although SPs were more satisfied with intervention than control residents. clinicaltrials.gov Identifier: NCT01808664.