Background and Purpose: Pro-androgenic substances are often used to treat muscle- or bone-related disorders. Their interactions with the classical androgen receptor (AR), however, trigger a series of undesirable effects. It would be of great benefit if the positive androgenic effects could be obtained by circumventing the classical AR. Experimental Approach: ZIP9 is a recently identified membrane-bound androgen receptor of physiological significance. Using in silico methods we identified and verified the extracellular localization of its androgen binding site and designed small peptides that fit in it without interacting with the AR. Six tetrapeptides with the best docking properties were synthesized and further investigated. Key Results: All tetrapeptides displaced T-BSA-FITC, a membrane-impermeable testosterone analog, from the surface of mouse myogenic L6 cells that express ZIP9 but not the AR. Silencing the expression of ZIP9 with siRNA prevented this labeling. All tetrapeptides act pro-androgenic: in L6 cells they stimulated the expression of myogenin, triggered activation of focal adhesion kinase, and prompted the fusion of L6 myocytes to syncytial myotubes. In human osteoblastic SAOS-2 cells that express AR and ZIP9 they reduced the expression of alkaline phosphatase and stimulated mineralization. These latter effects were prevented by silencing ZIP9 expression, indicating that the osteoblast/osteocyte conversion is exclusively mediated through ZIP9. Conclusions and Implications: Our results demonstrate that the synthetic tetrapeptides, by acting as ZIP9-specific androgens, have the potential to replace testosterone or testosterone analogs in the treatment of bone- or muscle-related disorders by circumventing the undesirable effects mediated through the classical AR.