Background: The human–animal bond is crucial for the proper development of animal-assisted therapy (AAT) sessions. However, in specific cases such as in AAT focused on psychiatric patients who are admitted to acute units, there is only a short period of time available to establish this bond. Objectives: To assess the degree of HAB generated after two AAT sessions in adolescents admitted to acute psychiatry units. Methods: A prospective multicenter cohort study aimed at adolescents aged 13 to 17 years admitted to the acute child and adolescent psychiatry unit of three hospitals. Intervention: Two weekly 1 h group sessions of AAT with therapy dogs. Main outcome: The Center for the Study of Animal Wellness Pet Bonding Scale Survey (CSAWPB). Results: One hundred and fourteen adolescents participated. A positive perception of the dog was generated, achieving a good level of bonding and attachment. Conclusions: The results obtained make it possible to affirm that a short period of time is enough to establish a good HAB between patients and therapy dogs. This study aims to contribute to the study of non-pharmacological interventions as a complement to pharmacological treatments in adolescents with mental health disorders.