Background: In 2015, the US Preventive Services Task Force (USPSTF) updated its primary care screening guidelines to recommend out-of-office BP testing with ABPM or HBPM for confirming the diagnosis of hypertension (HTN) in adult patients with high office BP. Methods: We applied the Behavior Change Wheel framework to develop a theory-informed strategy for increasing the uptake of the USPSTF HTN screening guideline. We then conducted a 2-arm cluster randomized trial in which 8 primary care practices (154 clinicians) in an ambulatory care network serving vulnerable communities in New York City were randomized to receive the implementation strategy (4 practices) or a wait-list control (4 practices). The strategy was implemented from October 2017 to March 2018, and consisted of: 1) clinician education about HTN screening recommendations; 2) clinician information on how to order ABPM and HPBM; 3) patient information on ABPM and HBPM; 4) nurse training on how to teach patients to conduct HBPM; 5) access to an ABPM testing service; 6) clinician feedback on out-of-office BP test ordering; and 7) embedded tools in the EHR to facilitate ABPM and HBPM ordering. The primary outcome was change in the proportion of patients completing ABPM or HBPM in the year before versus year after implementation. Results: There were 1069 eligible patients (mean age 53±16 years, 67% women) with high office BP but no diagnosis or treatment for HTN. In implementation practices, the proportion of patients with out-of-office BP test ordering increased from 0.5% in the year before implementation to 4.0% in the year after implementation (p<.001) whereas test ordering did not change in control practices (3.1% to 2.8%, p=0.66); p<.001 for interaction. Similarly, out-of-office BP test completion increased from 0.5% to 3.0% (p<.001) in implementation practices whereas test completion did not change in control practices (2.2% to 2.0%, p=0.76); p<.001 for interaction. Conclusions: A theory-informed implementation strategy increased out-of-office BP testing in adult primary care patients being screened for HTN. Yet, out-of-office BP testing in the context of HTN screening remained low in both implementation and control practices, suggesting a need for more potent implementation strategies.