Measurement of physical activity (PA) during routine care is currently not performed in most health systems. An “exercise vital sign” (EVS) is routinely assessed and entered into the electronic medical records at Kaiser Permanente. The purpose of this article is to examine the construct validity of the EVS for measuring PA in patients with chronic obstructive pulmonary disease (COPD). Patients with COPD who had a history of a COPD exacerbation requiring an admission, observational stay, or emergency department visit in the previous 12 months were included in the study. Baseline data of patients with COPD who were included in a pragmatic trial of a PA coaching intervention were used. Patients were classified as being inactive (0 min · wk−1 of moderate to vigorous PA), insufficiently active (1–149 min · wk−1), and active (150+ min · wk−1) using all available EVS data in the 12 months before identification for the study. The relationship between the EVS and standardized measures of COPD disease severity, functional performance, quality of life and symptoms were examined to establish construct validity. Because of the variable data collection scheme, data were available for 172 to 1834 patients. Patients with higher levels of PA, as categorized by the EVS, tended to have fewer comorbidities and less airway obstruction, used less supplemental oxygen, had higher 6-min walk test distance, and had more objectively measured PA, higher quality of life, and lower depressive symptoms compared with inactive and insufficiently active patients. In conclusion, the EVS provides a valid proxy measure of PA in patients with COPD and may be an efficient measure to assess PA during routine care compared with lengthy questionnaires or objectively measured PA.