Internet addiction (IA) is prevalent among adolescents and imposes a serious public health threat. Familial risk and protective factors may co-exist and interact with each other to determine IA. We conducted a cross-sectional survey among 9,618 Secondary 1 to 4 students in Hong Kong, China. About 16% of the surveyed students were classified as Internet addicted; nearly one third of them perceived that at least one of their family members had IA (FMIA). We found that FMIA was a risk factor (multivariate odds ratio [OR] = 2.04), and perceived family support was a protective factor (multivariate OR = 0.97) of IA. We also found a significant risk-enhancement moderation effect between these risk and protective factors, that is, the risk effect of FMIA increased with perceived family support. The finding highlights that family-based interventions, which modify familial risk and protective factors, should be effective for adolescent IA, but caution is required about potential risk-enhancement moderations between such factors.