Wearable blood pressure (BP) monitoring devices which measure BP levels accurately both in and out of the office are valuable for hypertension management using digital technology. The authors have conducted the first comparison study of BPs measured by a recently developed wrist-worn watch-type oscillometric BP monitoring (WBPM) device, the "HeartGuide," versus BPs measured by an ambulatory BP monitoring (ABPM) device, A&D TM-2441, in the office (total of 4 readings alternately measured in the sitting position) and outside the office (30-minutes interval measurements during daytime) in 50 consecutive patients (mean age 66.1 ± 10.8 years). The 2 BP monitoring devices were simultaneously worn on the same non-dominant arm throughout the monitoring period. The mean difference (±SD) in systolic BPs (average of 2 readings) between WBPM and ABPM was 0.8 ± 12.8 mm Hg (P = .564) in the office and 3.2 ± 17.0 mm Hg (P < .001) outside the office. The proportion of differences that were within ±10 mm Hg was 58.7% in the office and 47.2% outside the office. In a mixed-effects model analysis, the temporal trend in the difference between the out-of-office BPs measured by the two devices was not statistically significant. In conclusion, the difference between the WBPM and ABPM device was acceptable both in and out of the office.