This study will have two parts as follows:

The PK Lead-in Phase of the study will evaluate the steady state pharmacokinetics (PK) and confirm the dose of sofosbuvir (SOF) in hepatitis C virus (HCV)-infected pediatric participants. The PK Lead-in Phase will also evaluate the safety and tolerability of 7 days of dosing of SOF+ribavirin (RBV) in HCV-infected pediatric participants.

The Treatment Phase will be initiated by age cohort after confirmation of age-appropriate SOF dosage levels. Participants from the PK Lead-in Phase will immediately rollover into the Treatment Phase with no interruption of study drug administration. The Treatment Phase will evaluate the antiviral efficacy, safety, and tolerability of SOF+RBV for 12 or 24 weeks in pediatric participants with genotype 2 or 3 HCV infection, respectively.