This study aimed to describe the development of the Therapist's Perceived Knowledge Scale on Gender Identity Diversity and to preliminary validate this scale by describing its psychometric properties. This research instrument was built based on existing literature and recommendations for instrument development. Initially, a 36-item scale was designed to assess openness and perceived knowledge about gender identity diversity in therapy. The content validation process involved 12 expert judges, leading to a refined 25-item scale. 57 Puerto Rican trans and non-binary people participated and completed the scale. The exploratory factor analysis revealed a one-dimensional structure, supporting a single factor called “perceived knowledge about gender identity diversity in therapy.” The final scale demonstrated excellent reliability (α = 0.978; Sα = 0.980; Ω = 0.979), indicating a strong internal consistency. This validated scale mainly helps to evaluate the perceptions of trans and non-binary Hispanic people about the knowledge that their therapists have about the diversity of gender identity.