INTRODUCTION: Adolescents and young adults with cancer experience high levels of fear of cancer recurrence (FCR), negatively impacting their lives. However, no measure has been validated worldwide to assess FCR levels among these young people. This study aims to validate the FCR7, a scale that measures FCR, for the Portuguese population of adolescents and young adults. METHODS: Ninety-two participants were recruited online. They were primarily women (83.7%) with a mean age of 26.01 years at recruitment and 19.38 years at cancer diagnosis, with a variety of cancer diagnoses and previous treatments. Most participants were no longer under active treatment (75%), and approximately 75.1 months had passed since their diagnosis. Fear of cancer recurrence, anxiety and depression levels, and quality of life were assessed. RESULTS: The results showed that FCR7 has good model fit and reliability. Concurrent and divergent validity were also confirmed, with FCR being positively related to anxiety and negatively associated with quality of life. A cut-off score was found, discriminating between clinical and non-clinical levels of FCR. Almost 70% of the participants experienced clinical levels of FCR. We conclude that FCR7 is a valid unidimensional scale to assess FCR levels among Portuguese adolescents and young adults. CONCLUSION: More research should be conducted to validate FCR measures to be used among adolescents and young adults across the globe. The existence of a valid and brief measure to assess FCR among this population in Portugal is an asset for national health professionals and researchers.