Pre-exposure prophylaxis for HIV (PrEP) is recommended for people who inject drugs (PWID). Despite their central role in disease prevention, willingness to prescribe PrEP to PWID among primary care physicians (PCPs) is largely understudied. We conducted an online survey (April-May 2015) of members of a society for academic general internists regarding PrEP. Among 250 respondents, 74% (n = 185) of PCPs reported high willingness to prescribe PrEP to PWID. PCPs were more likely to report high willingness to prescribe PrEP to all other HIV risk groups (p's < 0.03 for all pair comparisons). Compared with PCPs delivering care to more HIV-infected clinic patients, PCPs delivering care to fewer HIV-infected patients were more likely to report low willingness to prescribe PrEP to PWID (Odds Ratio [95% CI] = 6.38 [1.48-27.47]). PCP and practice characteristics were not otherwise associated with low willingness to prescribe PrEP to PWID. Interventions to improve PCPs' willingness to prescribe PrEP to PWID are needed.