The development of biomaterials for application in advanced therapies requires thorough characterization of its biological behavior, which ultimately entails in vivo compatibility and performance assays. Electrospun fiber membranes of poly(L-lactic acid) (PLLA) and fish gelatin blends were produced and characterized, coupling the biomechanical features of PLLA with gelatin (GEL) biocompatibility. Fiber diameter was not affected by polymer blending, whereas the swelling degree increased with increasing GEL contents for values up to 566 ± 13%, behaving as a superhydrophilic material. Human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) adhesion was favored in the PLLA-GEL membranes, and cell viability was not affected after 7 days in culture. Membranes were then evaluated for in vivo biocompatibility through subcutaneous implantation in a rat model, for up to 15 days. No significant differences between the biological behavior of PLLA, PLLA-GEL, and GEL electrospun membranes at 15 days postimplantation were verified, with attained inflammation scores supporting an acceptable tissue response, deeming them fit for further biological assays. This work demonstrates that fiber blends of PLLA and GEL present promising in vitro and in vivo characteristics to be explored for tissue engineering.