Peptides and proteins are increasingly available for different therapies. However, adequate drug delivery systems need to be developed to enable the optimal clinical application. In this study, an in situ forming implant has been developed using poly(glycerol adipate)-g-oleate (PGA-g-Ox). After the synthesis and characterization, a directly injectable, solvent-free system has been obtained. It forms a depot immediately upon subcutaneous injection using a short, narrow cannula, with no need for a gelation time. Following that, the viscous polymer formulated with leuprolide acetate has shown a controlled drug release profile over four weeks. In vitro cytotoxicity assay carried out on 3T3 mouse fibroblast cells have presented good biocompatibility. Finally, a preclinical study, conducted on SKH1 nude mice and observed by non-invasive fluorescence imaging, has evidenced the influence of the viscosity and shape of the implant on the model drug diffusion.