Although synthetic polymers may have suitable physicochemical properties for biomedical applications, biological properties are generally lacking. Poly(ethylene glycol) (PEG) is a frequently used polymer for the preparation of hydrogels. Due to its hydrophilic character, however, cellular interactions with PEG hydrogels are minimal or absent. To improve the cell adhesive properties of PEG hydrogels, we developed hybrid hydrogels based on PEG and the natural polymer gelatin. PEG dimethacrylate (PEG-dMA) and gelatin methacrylate (GelMA) macromers were prepared, which were photo-crosslinked in water in different ratios (75:25, 50:50 and 25:75 % (v/v)). The obtained hybrid networks showed macrophase separation, which could be prevented by photo-crosslinking in 0.5 % (v/v) acetic acid in water. The toughness of 50:50 % PEG-dMA:GelMA hydrogels prepared in 0.5 % acetic acid was 2.5 times higher than that of single polymer hydrogels made of PEG-dMA or GelMA. Hybrid hydrogels crosslinked in 0.5 % acetic acid supported the proliferation of human mesenchymal stem cells to the same extent as compared to 100 % gelatin hydrogel, whereas the cells did not proliferate on 100 % PEG hydrogel. In conclusion, our results show that both the cell adhesive and mechanical properties of a photocrosslinked PEG network can be improved by incorporation of gelatin in the network.