Diet and fitness applications (apps) are marketed as health tools. Emerging research suggests that these apps may be linked to disordered eating symptomology. This systematic review aimed to collate and synthesise research examining the relationship between diet and fitness app use and disordered eating, including body image concerns and compulsive exercise. Additionally, it sought to identify how app type, frequency of use, and other contributing factors may influence the relationships investigated. A systematic search of peer-reviewed and published journal articles was undertaken using five electronic databases (Medline, PsycINFO, ProQuest, Scopus, and Web of Science). Thirty-eight articles were included. Disordered eating symptomology was higher in young adults who use diet and fitness apps and among those who use them more frequently, than non-users. Qualitative research indicated unintended consequences of this type of app use (e.g., pressure to meet goals and guilt). No causal conclusions could be drawn. Cross-sectional evidence suggests that the use of diet and fitness apps is associated with disordered eating, body image concerns and compulsive exercise. Future research should attempt to disentangle potential differences between app types and features, investigate potential risk and protective factors, and extend work to under researched populations (e.g., adolescents).