IMPORTANCE The incidence of human papillomavirus (HPV)-related cancers is more than 35 000 cases in the United States each year. Effective HPV vaccines have been available in the United States for several years but are underused among adolescents, the target population for vaccination. Interventions to increase uptake are needed. OBJECTIVE To evaluate the effect of a 5-component health care professional HPV vaccine communication intervention on adolescent HPV vaccination. DESIGN, SETTING, AND PARTICIPANTS A cluster randomized clinical trial using covariate-constrained randomization to assign study arms and an intent-to-treat protocol was conducted in 16 primary care practices in the Denver, Colorado, metropolitan area. Participants included 188 medical professionals and 43 132 adolescents. INTERVENTIONS The 5 components of the intervention were an HPV fact sheet library to create customized information sheets relevant to each practice's patient population, a tailored parent education website, a set of HPV-related disease images, an HPV vaccine decision aid, and 2½ hours of communication training on using a presumptive vaccine recommendation, followed by motivational interviewing if parents were resistant to vaccination. Each practice participated in a series of 2 intervention development meetings over a 6-month period (August 1, 2014, to January 31, 2015) before the intervention. MAIN OUTCOMES AND MEASURES Differences between control and intervention changes over time (ie, difference in differences between the baseline and intervention period cohorts of patients) in HPV vaccine series initiation (Ն1 dose) and completion (Ն3 doses) among patients aged 11 to 17 years seen at the practices between February 1, 2015, and January 31, 2016. Vaccination data were obtained from the practices' records and augmented with state immunization information system data. RESULTS Sixteen practices and 43 132 patients (50.3% female; median age, 12.6 years [interquartile range, 10.8-14.7 years] at the beginning of the study period) participated in this trial. Adolescents in the intervention practices had significantly higher odds of HPV vaccine series initiation (adjusted odds ratio [aOR], 1.46; 95% CI, 1.31-1.62) and completion (aOR, 1.56; 95% CI, 1.27-1.92) than those in the control practices (a 9.5-absolute percentage point increase in HPV vaccine series initiation and a 4.4-absolute percentage point increase in HPV vaccine series completion in intervention practices). The intervention had a greater effect in pediatric practices compared with family medicine practices and in private practices compared with public ones. Health care professionals reported that communication training and the fact sheets were the most used and useful intervention components. CONCLUSIONS AND RELEVANCE A health care professional communication intervention significantly improved HPV vaccine series initiation and completion among adolescent patients. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT02456077