Although HIV pre-exposure prophylaxis (PrEP) effectively and safely prevents HIV among adolescents, uptake of PrEP is low. Adolescents must have primary care providers (PCPs) prescribe them PrEP, making PCPs critical actors in PrEP delivery. However, research has primarily investigated determinants of PCPs' intention to prescribe adolescents PrEP rather than the determinants of performing the behavior itself. We examined the demographic, clinical practice, and implementation determinants of PCPs previously prescribing PrEP to adolescents. PCPs were recruited from a national Qualtrics panel of licensed medical providers in the United States from July 15-August 19, 2022. The Theoretical Domains Framework informed the implementation determinants measured. A multivariable logistic regression was used. PCPs who were more knowledgeable of the CDC guidelines (aOR 2.97, 95% CI 2.16-4.10), who were assigned male at birth (aOR 1.64, 95% CI 1.03-2.59), and who practiced in the Western region (aOR 1.85, 95% CI 1.04-3.30) had greater odds of prior prescribing adolescents PrEP. Provider-based educational interventions should be designed, implemented, and tested to encourage PCPs to prescribe PrEP to eligible adolescents.