Widespread adoption of electronic health records (EHRs) in medical care has resulted in increased physician documentation workload and decreased interaction with patients. Despite the increasing use of medical scribes for EHR documentation assistance, few methodologically rigorous studies have examined the use of medical scribes in primary care. To evaluate the association of use of medical scribes with primary care physician (PCP) workflow and patient experience. This 12-month crossover study with 2 sequences and 4 periods was conducted from July 1, 2016, to June 30, 2017, in 2 medical center facilities within an integrated health care system and included 18 of 24 eligible PCPs. The PCPs were randomly assigned to start the first 3-month period with or without scribes and then alternated exposure status every 3 months for 1 year, thereby serving as their own controls. The PCPs completed a 6-question survey at the end of each study period. Patients of participating PCPs were surveyed after scribed clinic visits. PCP-reported perceptions of documentation burden and visit interactions, objective measures of time spent on EHR activity and required for closing encounters, and patient-reported perceptions of visit quality. Of the 18 participating PCPs, 10 were women, 12 were internal medicine physicians, and 6 were family practice physicians. The PCPs graduated from medical school a mean (SD) of 13.7 (6.5) years before the study start date. Compared with nonscribed periods, scribed periods were associated with less self-reported after-hours EHR documentation (<1 hour daily during week: adjusted odds ratio [aOR], 18.0 [95% CI, 4.7-69.0]; <1 hour daily during weekend: aOR, 8.7; 95% CI, 2.7-28.7). Scribed periods were also associated with higher likelihood of PCP-reported spending more than 75% of the visit interacting with the patient (aOR, 295.0; 95% CI, 19.7 to >900) and less than 25% of the visit on a computer (aOR, 31.5; 95% CI, 7.3-136.4). Encounter documentation was more likely to be completed by the end of the next business day during scribed periods (aOR, 2.8; 95% CI, 1.2-7.1). A total of 450 of 735 patients (61.2%) reported that scribes had a positive bearing on their visits; only 2.4% reported a negative bearing. Medical scribes were associated with decreased physician EHR documentation burden, improved work efficiency, and improved visit interactions. Our results support the use of medical scribes as one strategy for improving physician workflow and visit quality in primary care.