Globally, 30% of new HIV infections outside sub-Saharan Africa involve injecting drug users (IDU) and in many countries, including Vietnam, HIV epidemics are concentrated among IDU. We conducted a randomized controlled trial in Thai Nguyen, Vietnam, to evaluate whether a peer oriented behavioral intervention could reduce injecting and sexual HIV risk behaviors among IDU and their network members. 419 HIV-negative index IDU aged 18 years or older and 516 injecting and sexual network members were enrolled. Each index participant was randomly assigned to receive a series of six small group peer educator-training sessions and three booster sessions in addition to HIV testing and counseling (HTC) (intervention; n = 210) or HTC only (control; n = 209). Follow-up, including HTC, was conducted at 3, 6, 9 and 12 months post-intervention. The proportion of unprotected sex dropped significantly from 49% to 27% (SE (difference) = 3%, p < 0.01) between baseline and the 3-month visit among all index-network member pairs. However, at 12 months, post-intervention, intervention participants had a 14% greater decline in unprotected sex relative to control participants (Wald test = 10.8, df = 4, p = 0.03). This intervention effect is explained by trial participants assigned to the control arm who missed at least one standardized HTC session during follow-up and subsequently reported increased unprotected sex. The proportion of observed needle/syringe sharing dropped significantly between baseline and the 3-month visit (14% vs. 3%, SE (difference) = 2%, p < 0.01) and persisted until 12 months, but there was no difference across trial arms (Wald test = 3.74, df = 3, p = 0.44).