(1) Background: This study investigates the experiences of prisoners with serious mental disorders who participated in a two-month dog-assisted therapy program within a Spanish prison. The aim was to understand the emotional and psychosocial impacts of the program on this population. (2) Methods: A qualitative descriptive design was used with purposeful sampling of sixteen prisoners (mean age 43.4) who completed the program. Data were collected through in-depth interviews and analyzed using thematic analysis to identify key themes. (3) Results: Participants reported a positive emotional impact, highlighting the significance of bonding, love, and reciprocal affection with the therapy dog. They emphasized the dog's unconditional nature as a source of comfort and connection. The importance of supportive relationships with the therapists who guided the sessions was also noted. Additionally, participants reflected on their motivations for joining the program and described progressive emotional changes over the course of their participation. (4) Conclusions: Dog-assisted therapy shows potential as a psychosocial intervention to enhance the prison experience for individuals with serious mental disorders. By fostering emotional well-being and positive connections, it may contribute to improved rehabilitation outcomes and better quality of life for this population.