Importance: Self-measured blood pressure (SMBP) with commercially available connected smartphone applications may help patients effectively use SMBP measurements. Objective: To determine if enhanced SMBP paired with a connected smartphone application was superior to standard SMBP for blood pressure (BP) reduction or patient satisfaction. Design, Setting, and Participants: This randomized clinical trial was conducted among 23 health systems participating in PCORnet, the National Patient-Centered Clinical Research Network, and included patients who reported having uncontrolled BP at their last clinic visit, a desire to lower their BP, and a smartphone. Enrollment and randomization occurred from August 3, 2019, to December 31, 2020, which was followed by 6 months of follow-up for each patient. Analysis commenced shortly thereafter. Interventions: Eligible participants were randomly assigned to enhanced SMBP using a device that paired with a connected smartphone application (enhanced) or a standard device (standard). Participants received their device in the mail, along with web-based educational materials and phone-based support as needed. No clinician engagement was undertaken, and the study provided no special mechanisms for delivering measurements to clinicians for use in BP management. Main Outcomes and Measures: Reduction in systolic BP, defined as the difference between clinic BP at baseline and the most recent clinic BP extracted from electronic health records at 6 months. Results: Enrolled participants (1051 enhanced [50.0%] vs 1050 standard [50.0%]; 1191 women [56.7%]) were mostly middle-aged or older (mean [SD] age, 58 [13] years), nearly a third were Black or Hispanic (645 [31%]), and most were relatively comfortable using technology (mean [SD], 4.1 [1.1] of 5). The mean (SD) change in systolic BP from baseline to 6 months was -10.8 (18) mm Hg vs -10.6 (18) mm Hg (enhanced vs standard: adjusted difference, -0.19 mm Hg; 95% CI, -1.83 to 1.44; P = .81). Secondary outcomes were mostly null, except for documented attainment of BP control to lower than 140/<90 mm Hg, which occurred in 32% enhanced vs 29% standard groups (odds ratio, 1.15; 95% CI, 1.01-1.34). Most participants were very likely to recommend their SMBP device to a friend (70% vs 69%). Conclusions and Relevance: This randomized clinical trial found that enhanced SMBP paired with a smartphone application is not superior to standard SMBP for BP reduction or patient satisfaction. Trial Registration: ClinicalTrials.gov Identifier: NCT03796689.