In the context of externalising behaviour problems, risk factor research (RFR) focuses on risk and protective factors of juvenile delinquency, which can pertain to individual, system, and societal levels. Several instruments aiming at measuring these factors have been developed, but a comprehensive research tool is missing. The aim of the present study was to develop and validate a questionnaire, the “Family, Peers, and Externalising Behaviour in adolescence” (FPEB) as a tool for assessing adolescents’ tendency of externalising behaviour, the quality of relation with their parents, and peer-relations. FPEB was administered to 835 Italian students (36.8% males, age M = 13.81, SD = 1.54) together with the Moral Disengagement questionnaire to test concurrent validity. Data about socio-demographics and school performance were also collected. An EFA (Promax rotation, subsample A, n = 444) resulted in a four-factor structure that was corroborated by a CFA (subsample B, n = 388). The factors were “externalising behaviour” (var 13.16%), “peer relations difficulties” (var 11.10%), “Family conflict” (var 8.32%), and “lack of family negotiation” (var 7.11%) and showed good internal consistency (all α ≥ 0.65). There were differences between males and females in the correlational patterns of the four factors. The FPEB factors also showed good concurrent validity: two of the four factors (“lack of family negotiation” and “externalising behaviour”) and the total score of the scale correlated with the “Moral disengagement scale”, whereas peer relation difficulties did not. Further analyses also showed gender differences (except for “peer relations difficulties”) and an association between students’ school performance and “externalising behaviour”, “family conflict”, and the total FPEB scores. We concluded that the FPEB is a tool that is potentially useful to assess risk and protective factors and to plan targeted interventions (focusing on the specific area). Limitations and suggestions for further improvements are also discussed.